The mothers of the titular Animorphs show this trope in spades towards the end of the series. Some examples:
The Diversion, where the Animorphs finally out themselves to their family, has several examples. Cassie's mother puts her body between her daughter and Ax, whom she thinks is dangerous, mutated wild animal. Rachel's mother grabs a spice rack and faces down what she believes is a full grown male grizzly bear to protect her two younger daughters (unaware said bear is her actually oldest). Tobias compares this to the way his own mother seems to haveabandoned him, but the book ends with her taking a Dracon beam in the back for him too.
Eva, Marco's mother, repeatedly denies opportunities to free herself from the torment of being Visser One's slave (which at one point had her pushed off a cliff and then tortured) in order to help prevent an all out war that would very likely claim the lives of her husband and son. Obviously, badassery is genetic in the Animorphs universe.
Visser One herself is a villainous example. She is a Puppeteer Parasite who conceived and delivered twins through a human host and considers them her own, though she was forced to give them up to maintain her cover. The whole reason she opposes an open war to conquer Earth is because it could kill billions, and those billions might include the only two humans she cares about.
This is Cersei Lannister's main motivation in A Song of Ice and Fire, and her only good trait. Joffrey, Tommen and Myrcella are the world to her, and to protect them from a prophecy she has interpreted as saying that they'll be crowned and die before she kicks the bucket, she will do anything. Including arranging the death of her husband's bastards any time she finds one, baby or not.
Catelyn Stark also turns into one of these, going so far as to free Jaime "the Kingslayer" Lannister in exchange for her daughters Sansa and Arya, and later she goes on a Knight Templar-like vendetta against the Freys because, among other things, they brutally killed her eldest son Robb.
Having found the man she believed to have kidnapped her son, Ce'nedra went psycho in his direction. She had to be restrained.
Do NOT threaten ANY of Beldaran's descendants or you will have a very pissed, very powerful Polgara making sure you have a VERY long time to regret it.
In John Varley's story The Bellman, Anna Louise Bach. Think that because she's in labor she can't defend her about-to-be-born child? Guess again. She's got a power drill.
An extreme example is found in Toni Morrison's Beloved, where Sethe, the main character attempts to kill her children in order to protect them from having to go back into slavery. She only succeeds with the titular Beloved.
Beowulf has among the earliest Mama Bears of literature. When the creature Grendel is killed by the hero, his mother attacks and makes her anger known.
In John Gardener's satire-novel Grendel, told from Grendel's point of view, he recalls a time when he was separated from his 'mama' as a child (he actually calls her that sometimes!), with his ankle trapped between two trees, unable to free himself. After a while of screaming for his mama - so loudly the ground rumbles - that a bull comes and attacks him. Why? Because Grendel was going after a newborn calf that he smelled, distracting him from getting home before dawn. After the bull gives up on trying to knock him out of the tree and gore him, Grendel encounters humans for the first time. Who eventually attack him. Just as Hrotgar's ax hits him across the shoulder, Mama comes in with such a fireball of fury, scaring off the humans...and maybe even Grendel himself. And she tears down the trees to make him fall out.
Subverted in The Bible, where Athaliah, queen mother of Ahaziah, responds to the killing of her son the king not by going after the killer...but killing her grandchildren and taking over the throne.
In John C. Wright's Chronicles of Chaos, while Echidna is never exactly safe, it is the death of her son that inspires her to come and slaughter every human being she finds.
A big, quite literal example can be found in Sherrilyn Kenyon's Dark-Hunter series. Particularly in any of the books that mention the Peltiers - A family of Katagari (animals who can turn into humans) bears. You do not mess with Nicolette 'Mama Lo' Peltier or her family. She will beat ten types of crap out of you before you have time to rethink your decision or write out your will.
In the Dear America book A Coal Miner's Bride: The Diary of Anetka Kaminska Americans were throwing rocks at Anetka and her family. She tried to ignore it until one hit her step-daughter at which point she says she became a mad woman like a mother cat.
Mrs. Roe in A Line in the Sand responds to a raid on the village by loading a musket. When her daughter asks if she's really going to shoot someone, she says she will if she has to: no one is going to hurt her family.
In the 4th Dexter novel, Dexter by Design, a bound and gagged Rita manages to knock her kidnapper into a running table saw in response to his threats to her children and husband.
This trope sees many uses in Terry Pratchett's Discworld:
Magrat in Carpe Jugulum who disposed of Countess Magpyr in a cold-hearted Mama Bear showdown. While she had a Beware the Nice Ones moment in every book prior to her daughter's birth, it was always a one-off thing triggered by a Berserk Button. In CJ, though, her determination to protect baby Esme makes her possibly the most in-control of the witches (at least in Granny Weatherwax's absence). As Agnes thinks, mothers aren't wet, they're only slightly damp.
Also invoked and deconstructed by Granny Aching in The Wee Free Men, where she teaches a valuable lesson with the aid of a Mama Sheep.
In Mort there's mention of a sacrificial goat that gave birth to twins just before the fatal part of the ceremony, and chased all the priests out of the Temple of Blind Io in defense of her kids.
It's mentioned in one of the books that one of the only creature Greebo ever backed down from was a vixen with pups (this is the Greebo who will chase bears up trees).
Before meeting Sam Vimes, the only thing Sybil Ramkin really cared about was looking after her swamp dragons. In Thud!, upon learning that the dwarf who just got incinerated by said swamp dragons was the fourth member of a team of assassins sent into their home to be killed or captured and the number remaining is unknown, she hands Sam one of the dragons and says "Coal him up."
Glenda in Unseen Academicals, towards pretty much everyone in her sphere. According to Vetinari note who should certainly know — her grandmother was the cook at the Assassins' Guild when he was at school there, and had a standing rule about never using her food to poison anyone, it's a family trait:
'That’s a Sugarbean woman for you, Drumknott, little domestic slaves until they think someone has been wronged and then they go to war like Queen Ynci of Lancre, with chariot wheels spinning and arms and legs all over the place.’
The narration of The Divine Comedy compares the resident Mentor Archetype, Virgil of Limbo, to a mother who is woken up by a fire and grabs her kid without pausing, putting his safety above her own. In the same way as that figurative Mama Bear, Virgil "snatched [Dante] up" and rushed out of the circle before any demon could scratch him.
Stephen King's Dolores Claiborne. She puts up with years of physical and mental abuse from her husband, because her main focus is giving her kids the best life she can by saving for each of them to go to college. But when she finds out that he's hurting the kids, he has to die. She's very methodical about it, arranging to make it look like an accident and ensuring that none of her children are home at the time it happens.
No matter the supposed heroics of the main cast, the prisoners' escape from Pax Tharkas in the first Dragonlance novel (Dragons of Autumn Twilight) would have ended in utter disaster if a certain old, insane, and technically evil dragoness hadn't come through for 'her' (adopted) children in the end and taken on the other (younger and stronger) dragon present all by herself.
The Dresden Files has Charity Carpenter, who is not only a badass with a sword, but fiercely protective of her kids, to the point where she scares even Harry.
There's Susan Rodriguez, who performs a Heroic Sacrifice for her daughter Maggie.
Hints of this with Harry's mother as well. Since Harry is quite a lot like her, this is almost a certainty.
Then there's Queen Mab. Now, she probably didn't give two shits about some two-bit players trying to cause trouble in her court (until they spilled blood), but trying to harm Sarissa didn't exactly earn her favor.
Titania almost kills Harry, despite the fact that he was trying to save the world and everything in it, because Harry killed Aurora in a previous book (for a good reason of course). She had been getting therapy, lucky for him and the world.
Eleven-year-old black belt Candy beats a grown man to death in Emergence, by David Palmer, when he makes the fatal error in judgement of launching a potentially deadly attack with a frying pan on her lifelong pet/sibling Terry (a hyacinth macaw), who is referred to as her "child substitute".
In the same book, a would-be rapist finds out about Mama Bear the hard way when Kim catches him with six-year-old daughter Lisa. Kim's biggest distress over this incident is how long it took to get the mess out of the carpet.
The Han Solo Trilogy details Han in his youth and early years as a smuggler, and the first book, Paradise Snare, shows that Chewbacca wasn't the first Wookiee that he developed a close bond with. Serving as a cook aboard the outlaw Garris Shrike's vessel, she came to see the orphan Han Solo as a son, and he reciprocated the feeling. At 16 years of age, Han attempted to strike out on his own, knowing that Shrike would use his abilities as a pilot and scam artist until Han either got caught or was no longer useful. After stealing some supplies, Han met with Dewlanna to say goodbye, but was met by Shrike and his men, and past experience made it clear what was coming. Dewlanna rose to Han's defense and beat up Shrike's men, viciously shattering his brother's arm, before being gunned down by the outlaw. Her sacrifice allowed Han to escape and grow into the man that everyone knows him as.
Tash Arranda in Galaxy of Fear is sometimes an odd example. She starts the series at thirteen years old, with a brother a year younger. Feeling Promoted To Parent, she does what she can to help him with their Orphan's Ordeal, but has understandable separation anxiety and abandonment issues. When her brother disappears, Tash believes a Hutt gang lord who threatened both of them before may have abducted him, and goes to beard him in his lair without hesitation. In many other books they're more of a Brother-Sister Team with each helping out the other.
Kirney Slane AKA Lara Notsil finds out in Mercy Kill that the new Wraith Squadron contains an alarming number of children or descendants of the original members. She makes Voort state that he will not recruit her children. Four times.
The Hank the Cowdog series brings us Sally May, mother of Little Alfred and Baby Molly. Unfortunately, Hank is quite often the target of her scorn for "corrupting" her children. Still, more than once she's stood up to some pretty serious dangers to keep her kids safe.
Gertie Cat, Though being thin and tired from constant nursing, manages to make a bull run for it's life when it tries to eat the haystack that her kittens are hiding in. She even manages to bring out Hank's Papa Wolf side. When Pete threatens them Hank (quite gladly) pitches him out into a downpour.
This happens in Harry Potter and the Deathly Hallows; when resident Dark Action Girl Bellatrix Lestrange nearly kills Ginny Weasley, Ginny's House Wife mother Molly goes ballistic and kills her in a matter of seconds with a well-placed curse at the chest. "NOT MY DAUGHTER, YOU BITCH!" quickly became the most-quoted line from the book. This is a double example: Bellatrix showed her "Too Dumb to Live" credentials by mocking Molly's recent loss of one of her sons, Fred, which only got the Action Momeven more pissed off than she already was. According to Word of God, Molly comes from a family of aurors, and her brothers Fabian and Gideon Prewett had been killed by Voldemort and his followers years earlier. Twin brothers F and G? Not likely a coincidence. The reaction could have been skill or sheer instinct — remember Molly's boggart, alias her biggest fear? Losing her family and friends. And, at the time, Molly (and everyone else) still thought Harry was dead, whom she viewed as a son. This was just one more reason for her to want to kick the Death Eaters' asses. Just how powerful does Molly turn out to be? Earlier, Professor McGonicall was ambushed by four Aurors firing Stunners at her chest, which knocks her out. Molly fires a Stunner at Bellatrix that causes immediate cardiac arrest. It's turned up even more in the film, in which the spell squeezes Bellatrix's chest for a moment, then disintegrates her.
This is the reason Harry survived his first encounter with Voldemort. His mother Lily was given the chance to flee but she chose to protect baby Harry with her life, which granted Harry magical protection. Notice that this was a young woman in her 20's without her only weapon and in front of one of the most powerful mages ever — and she chose death over handing her baby to the Death Eaters.
Narcissa Malfoy, Draco Malfoy's mother, proves that Even Evil Has Loved Ones: in the seventh book she lies to Voldemort so she can reunite with her son Draco. She may be a pureblood supremacist with the mindset of Screw the Rules, I Have Money!, but she is willing to cross the mightiest dark wizard of all time to ensure her son's safety. Extra marks for successfully lying to the greatest mind-reader of the current age.
McGonagall has shades of this when it comes to her students, as shown when she defends Harry from Snape. Also, in a castle controlled by Death Eaters, she, despite being noted as old, continues to stand up to people who are known for their hobby of torturing and killing people they don't like, all for the sake of her students. And in the final book when she learns Hogwarts is about to be attacked by Voldemort's forces, she takes charge to protect her students.
In another Diana Wynne Jones novel, Black Maria, Mig's mother becomes very angry when she finds out that Aunt Maria had turned her son into a wolf and tried to get the town to shoot him. Later, when Mig is being held captive in the orphanage (which is essentially used to brainwash the town's children), her mother storms in and demands that the people in charge tell her where her daughter is. All we see is the mother dragging the head of the orphanage and shaking her for locking Mig up, but it's implied that she did more. The children in the orphanage are delighted and cheer her on.
In Death: Areena Mansfield murdered Richard Draco in Witness In Death. Why? Because he was having sex with their daughter Carley. She had told him that she was their daughter because she thought it would turn off his interest in her. Instead, he went and did it, knowing that he was committing incest. He bragged about it to her and wanted to have a threesome composed of him, Areena, and Carley. If you do not consider this a good enough reason for her to go Mama Bear, then you clearly have no soul!
Darcy in Quicksand House is a pre-teen, deranged, stab-happy version of this trope. She doesn't care much about most people, but she's extremely protective of her adopted little brother / son Drool, and even dotes on Leech despite Leech being so young that she should still register as unbearably ugly to adult humans (It Makes Sense in Context). The nannies are also all like that - threaten a nanny's charges, and she'll pull off her own leg and hit you with it in their defense.
Inverted in E. E. “Doc” Smith's Children of the Lens, in which the heroine's daughter wreaks havoc with the mind of an enemy agent in order to force that agent's co-operation with her mother. In fact the trope plays almost straight in-universe because the agent views the mother as automatically more capable than the daughter (not knowing that the inverse is true here) and alters her perceptions and behaviour accordingly.
The cover blurb from Holly Lisle's novel Minerva Wakes notes: "Three rules govern life: Never give up on your dreams. Always do what is right. And NEVER mess with the Mommy."
If those cadets come near my Year Sevens again, I will maim them.
Tate, when she finds out her neighbor left her daughter with a child molester.
In Penny from Heaven, Penny relates an incident in which her grandmother threatened Bobby the neighborhood bully with a meat cleaver after he hit Frankie.
In Percy Jackson and the Olympians, Sally Jackson, the loving and nurturing mother of Percy, was shown fighting in the last battle using a stolen shotgun from the police.
Yanaba Maddock, the protagonist of the Petaybee books, becomes this in the spinoff series The Twins of Petaybee.
Sidney Sheldon's Rage of Angels reveals lawyer Jennifer Parker to be a version of this, though she has to work indirectly. When her son is kidnapped, she knows time is of the essence, so she [calls on the Mafia prince who's been lusting after her — whom she knows to be very bad news from personal experience — and asks him to do whatever he can to bring the child back alive, and when asked what to do with the kidnapper, she says "Kill him!" He mobilizes his forces to track them down, the child is rescued, and he kills the man himself. The two adults become lovers after this, setting up the remainder of the book.
Coupled with an instance of Good Stepmother in Ragnar Lodbrok and His Sons: When her stepsons Erik and Agnar are killed by Eystein Beli, Aslaug drives on her biological sons to avenge them, and personally leads an army to battle Eystein, who is eventually defeated and killed.
In The Red Tent, when she finds out that her husband Laban had been fondling Leah and Zilpah, Adah proceeded to beat the ever-living crap out of him. When he was just inches away from dying from the resultant injuries, Adah swore that if he did it again, she would not only beat him up again, but that she would call upon the Powers That Be and have them give him some Laser-Guided Karma in the form of disease and impotence. Laban then made sacrifices to the gods, had a statue of one of their chief goddesses made, and bought Adah and his daughters jewelry...and never touched the girls again. She also brought it up again, when he had made a deal with Jacob to marry Rachel off to him, because Rachel had not yet menstruated. In all fairness, Laban didn't know that, but the threat was enough to get him to comply with his wife's wishes, and extend the time Jacob worked for him.
Done to absolutely terrifying effect in Agatha Christie's short story "The Last Seance." In the story, a young woman named Simone who is genuinely gifted with the psychic ability to channel the dead plans on finally retiring, as her power exhausts her mentally and physically. However, she agrees to perform one last seance for a woman named Madame Exe, who lost her only daughter Amelie and still mourns her. When Madame Exe shows up, she is reminded that she must not touch whatever manifestation appears for fear of harming Simone. But when the young woman does manage to physically channel Amelie's ghost, Madame Exe goes absolutely ballistic and grabs the child, despite Simone's screams. This ends up killing the psychic (she manifested the spirit from her own body, and breaking the link essentially ripped her in half), but Madame Exe doesn't care. All she wants is Amelie, and she is willing to defy death itself if it means getting her back.
Inverted in Skulduggery Pleasant: Death Bringer when Valkyrie hears her mother was attacked by a mugger and breaks into his cell to beat him up.
In The Secret of Platform 13,Larina Trottle is a villainous example, using her vast resources to move heaven and earth to protect her son from kidnappers. (Despite having kidnapped a child herself to get a son; these new kidnappers, in fact, are trying to restore him to his rightful family.) She also plots to destroy the life of her servant boy, Ben, for the simple crime of being smarter and more generally likeable than her little darling. (For added hypocrisy, Ben was the kid she kidnapped and planned to raise as her son, until she got pregnant and tossed him aside.)
Although Sarah Heap usually proves that Adults Are Useless, she can become very vocal if her family or any of her children are threatened in any way, as Marcia Overstrand had to find out the hard way in Flyte:
Queen Cerys's, Jenna's mother, job is to keep her daughter safe from harm, as she does by throwing Queen Etheldredda out of the Queen's Room in Physik.
In Michael Flynn's Spiral Arm novel On the Razor's Edge, Bridget calls in the Hounds to get aid: they are going after her daughter into enemy territory. Later, one Shadow is surprise at how many Hounds followed; another is not, because she had stolen the cub from the Mama Bear.
Dagmar Obscura as an example of an Action Girl ascended to Action Mom. Don't dare to touch her beloved daughters because it doesn't matter if she's dead, she will find the way to kick your ass. Even her husband knows this.
Placida is this for Luisa after Dagmar dies.
Kadiri Miyamoto is this for everyone on her shrine. She's devasted before the death of one of her nuns.
A rarely discussed, but still notable, example would be Mara Jade Skywalker, from the Star WarsExpanded Universe. Probably most notable in Sacrifice, where her worry for her son (Ben) convinces her to go hunt and attempt to kill the two Sith Lords whom she thinks are plotting to kill him. She's partially right in that respect, and this leads to a VERY nasty brawl. Do not mess with a Mama Bear who's also a trained assassin. It will not end well for you. Unless you're Darth Caedus, and even then, he barely escaped.
Meg in Stork Raving Mad. Think that because she's in labor with twins she can't defend her about-to-be-born children? Guess again. She's got a hat rack.
John Wyndham's short story Survival, in which one of the stranded passengers resorts to extreme measures to make sure her baby doesn't starve.
Bliss, Susan's hippie mother, takes someone out with a statue of Buddha to protect her full grown daughter in Sweetheart.
Rachel O'Neal, in Teetoncey by Theodore Taylor, is this to a young shipwreck victim whom she nicknames "Teetoncey". Do not start with Rachel O'Neal. You will not win. That includes you, Atlantic Ocean.
In The Tenant of Wildfell Hall, Helen's fears for her son and not her own misery force her to take him and run away from her husband. While this may seem like logical behavior now, at the time the novel was written this was illegal, scandalous, and unheard of.
Time Scout: Ianira. In Wagers of Sin, the fact that Skeeter was trying to rescue the father of her children gave her words extra weight, for both uptimers and downtimers.
Lady Svetlana in Tranquilium is very much this after giving birth in the beginning of Part Two. She goes out of her way to protect her little boy from revolutionary terror, war and a scarily ruthless conspiracy based in another dimension, though she got help along the way; for all her otherissuesandwhatnot, the safety of her child clearly becomes an overriding concern for her for the rest of the book. This status of hers is also helped somewhat by the fact that she is actually pretty good at fist-fighting thanks to the training she underwent with her father's men (her father was a captain).
In Unfinished Tales, Christopher Tolkien describes a Worthy Opponent version of this trope. In the wars between Gondor and the Wainriders, a revolt is arranged among some of the Wainriders slaves while the men are away on campaign:
"But most of them perished in the attempt; for they were ill-armed, and the enemy had not left their homes undefended: their youths and old men were aided by the younger women, who in that people were also trained in arms and fought fiercely in defence of their homes and their children."
Also, in the Book Of Lost Tales version of the Fall of Gondolin, Idril is described as fighting 'like a tigress' to protect her son Eärendil from Maeglin. She fails, but when Papa Wolf Tuor turns up...
Cordelia Vorkosigan is an off-worlder who is mostly bored by the Byzantine politics of her husband's home planet Barrayar. Until a civil war puts her baby (in a high-tech incubator) in danger. Then she single-handedly defeats a usurping ruler and ends the war. And brings back the usurper's head in a shopping bag to make her point clear.
Princess Kareen Vorbarra makes a very credible attempt to kill her unwanted lover when she realizes the man is a threat to the life of her five-year-old son, Gregor. She fails, but not for lack of trying.
Lois Mc Master Bujold's later novel Captain Vorpatril's Alliance the controlled, undemonstrative half-haut Baronne Cordonah reveals her inner Mama Bear pinning the treacherous ally who has endangered three of her children to the wall.
Many queens in Warrior Cats behave like this whenever their kits are in danger or bullied. Several examples include Yellowfang, Leafpool, Sasha, and Sandstorm.
Averted with Rainflower, the mother of Crookedstar and Oakheart, who cruelly neglects Crookedstar all because he broke his jaw. Thus, it's Shellheart who brings out his Papa Wolf roll.
World War Z has a woman who goes into a blind rage when a zombie tries to get her daughter. Her children told her later that she had ripped its head off with her bare hands. There was also the nun who held off a zombie horde for quite some time with a large iron candlestick, protecting the children from the local Catholic school.
Nita Callahan's mother in The Wizard's Dilemma turns into one of these when faced with the prospect of the Devil-equivalent taking Nita's soul. Her line is "She is still my daughter, and she does not have my permission!"
In High Wizardry, Dairine Callahan becomes the "mother" to a race of sentient silicon lifeforms, and a threat to them (as well as Nita and Kit) prompts her to tell the Devil-equivalent, "Touch them and you're dead meat." Shortly after, when a powerful ally appears to help out, the ally's gender turns out to be female, surprising the protagonists, said ally then comments "Men will fight bravely and be heroes, but for last-ditch defense against any odds . . .get a Mother."
Averted in Frostflower and Thorn because while Thorn mounts a daring rescue for her biological son Starwind she's doing it for Frost's sake, not his, and while Frostflower would give her life for her son she would not take a life for him, especially since she knows he won't be harmed by the priests.
Mere Lessard of Sacré Bleu nearly brains the titular muse with a crepe pan when she believes that Bleu is endangering her son. She earlier gives a few gruesome examples of what she will do to "Juliette" if Lucien comes to any harm whatsoever.
Jodi Picoult's novel Perfect Match is about a woman who shoots and kills a man who she believes molested her five-year-old son. Tragically, he turns out to have been innocent.
In M.T. Anderson's novel He Laughed with His Other Mouths, Dolores Dash battles an evil alien who has her son under mind-control. She causes a building to collapse on the alien.
In The Fight For Home Arianna goes out on her own to engage an approaching enemy fleet. Said fleet wishes to kill her son and his crew and she will only accept either the commander being dead or the fleet going through her dead body to get to her son.
Despite her hatred for children, Imogen Herondale in City of Ashes acknowledges this trope, coming to the conclusion that Valentine would do anything for his son like she would.
Lilith considers Jonathan her "son" due to him being tainted by her blood, and takes measures to protect him throughout the second trilogy.
In 1632, Wallenburg's Croatian mercenaries quickly learn that it is a very bad idea to attack a town whose defenses include two very formidable female fighters and a female tactical expert, all of whom are currently pregnant (one with her second child, her first also being in town). Right, Gretchen, Julie, and Rebecca? For bonus Too Dumb to Live points, they go after the school, where most of the town's kids are being protected.
Janine Hathaway from Vampire Academy is very protective of her daughter, and deeply cares about her safety and well-being. She is very intimidating to Rose's would-be boyfriends.
His Dark Materials: Don't mess with ma Costa. When a man accused the missing Billy of leaving, he didn't get a chance to finish his sentence before he had to run from her physical and verbal attacks.
Mrs Coulter may be a horrible and mostly absent mother and loves watching children become separated from their souls (most died quickly), but leave her own daughter alone! In the end, she ultimately sacrificed herself and her afterlife to take down the ultimate angel who wanted her daughter's death.
Cassandra Kresnov discovers that she's this after she befriends a trio of war orphans in Operation Shield. Considering that she's an Artificial Human supersoldier who makes the Terminator look about as threatening as a Robosapien doll, any threat to her kids quickly becomes a Mook Horror Show.
Another Stephen King instance: In "A Good Marriage", Darcy discovers that her husband is a Serial Killer. She realizes that her children will run into serious problems with careers and personal lives if this becomes public knowledge, so she solves the problem herself by killing her husband in a way that looks accidental, to keep his activities secret while putting a stop to same.
Journey to Chaos: When Sathel caught her daughter's kidnapper, the only reason she didn't kill him then and there was because he was about to turn himself in to a Justice Station. Instead, she subjected him to years of poison-induced illness, nightmarish hallucinations, and horrible itching.
In Ratman's Notebooks, the title character's relationship with the Swarm of Rats starts when his mother orders him to drown the rodents that have infested her garden, but he's too impressed by the mother rat's determined efforts to rescue her babies to go through with it.
Erienne in spades. Most of the book Nightchild is devoted to her role as this. Friend or foe, warrior or mage, nothing will prevent her from protecting her and Denser's daughter.
Jason Porath of Rejected Princesses fame is about to come out with a second book called Tough Mothers that will focus on tough mothers from history. The cover features a bear on her hind legs with a cub hiding behind her.