AUDevil May Cry fic "Like Father, Like Son" has Dante exhibiting his demonically-protective side when he finds his son Nero injured courtesy of a police officer with temper problems, and when Nero is later threatened by members of the mafia. The second time Nero gets threatened by the son of a mafia head, a partial Devil Trigger sends the man leaving with tail between legs. And when Dante finds the demon Baul, who had all but put Nero near Death's door... Cue Unstoppable Rage and Roaring Rampage of Revenge with a dash of There Is No Kill Like Overkill.
In Harry's New Home, Snape will go to any lengths to protect his ward Harry, even if it included breaking rules or facing the Dark Lord and Dumbledore (if necessary) to do it.
In Supernatural's Crossing the Line, John Winchester kills a human being for the first time because it was heavily implied said person had molested/sexually assaulted a ten-year-old Sam.
In the Ultimate Re Imaginings, Clint is this towards Joey as he once nearly killed the boy's actual father for putting him in the hospital and breaking his arm when Joey had been twelve.
In Lord of the Rings fanfics such as An Eye For An Eye or The Roots of Evil, it is a sure way to trigger Legolas's Berserk Button is by hurting Aragorn in any way, shape or form. It is the same with his older adopted brothers Elladan and Elrohir, who must have gotten it from their father Elrond. All four elves have no hesitation to resort to killing anyone who dared lay a hand on Aragorn. And those times when they found that Aragorn was tortured and near death ... well, to say it did not go well with those people is putting it lightly.
Child Of The Storm has Thor, God of Thunder and Lightning (not Reason and Understanding), is the father of the titular child, Harry and a fairly epic Papa Wolf. Raise a hand to Harry, even threaten him, and you will face the full wrath of the Thunderer, usually delivered via a supersonic Mjolnir and a record breaking bolt of lightning or several thousand.