Ma'ar/Mornelithe Falconsbane started as being a Well-Intentioned Extremist, but quickly became the Big Bad, and the final showdown between him and the Big Good Urtho caused the Cataclysm, which was so powerful that it actually echoed through time. Though defeated, he didn't actually die, as he had set up a Grand Theft Me so that he could take over the body of any his descendants once they started showing signs of magical ability. Falconsbane kept doing this over the centuries until finally the main characters in the Mage Winds trilogy figured out what was going on and destroyed him.
Ancar kills his father in order to take the throne of Hardorn, tortures Queen's Own Talia (who was there as part of a diplomatic mission) more for kicks than to get information out of her, and then has his mages mind-control the conscripted troops so that they'll fight. He then drains his land to further power his spells, and kills off even his distant cousins to prevent anyone from overthrowing him.
Hadanelith, a minor villain who appears in The White Gryphon, possesses the Gifts of Empathy and Mindhealing, but uses them to serve his own sadistic desires rather than to aid people. When first discovered, he is found to have been using his gifts to warp the minds of women until they live only to serve him in slavery, sexually and otherwise. He is exiled from the city of White Gryphon under the relatively loose laws and customs of their society. Rather than die in the wilderness, he makes his way south to the Haighlei kingdoms and is recruited by the Evil Chancellor to assassinate high ranking members of their society in order to frame the delegation from White Gryphon. In doing so, he is permitted to indulge his sadistic fantasies and accordingly tortures his victims before killing them.
In Intrigues, Bear (an aspiring Healer who is supposedly the protagonist's best friend) baselessly accuses Mags of murder and threatens to beat him to death. Mags is a 14-year-old child. And despite the fact that Bear's abuse almost drove him to suicide, they're buddies again in the end of the book, with no apology or effort on Bear's part.
In Storm Breaking, Elspeth takes sadistic joy in the suffering Tremane inflicts on himself so he can protect his countrymen. Mind that Elspeth is literally marked out by the powers that be as a Good Person.
Why does Firesong become obsessed with a lifebond when the Mage Storms and their effect on the land are driving him insane? One of the other books reveal that lifebonds usually/often form when someone extremely powerful is in danger of going insane: the lifebond anchors them. As a Healing-Adept, it's probable that he subconsciously knew what was happening to him and what he needed - it's not made clear whether lifebonds are divine intervention, something the more powerful person does subconsciously, or both.
In By The Sword it's revealed that Valdemar has forgotten the Binding Ancient Treaty between it and Rethwellen. Sounds strange, and then one remembers that Selenay was a teenager when she became queen and her father died in the prime of his life. Even if her grandfather was part of how the treaty was formed, between her sudden ascension to the throne and all the other things that happened along the way, a treaty like that being forgotten is very justified! Additionally, while the agreement itself was non-magical, the circumstances that led to it directly involved a mage (Kethry) and magic, so the Valdemaran compulsion against magic was likely also working against people's memory of the whole business and the treaty that resulted from it.
when reading the Fridge Brilliance entry above about Firesong, and his lifebond obsession. Now think about Lavan Firestorm, who turned out to be lifebonded to his own Companion (which also made both of them effectively Asexual). When Lavan lost her, he went utterly insane, and turned himself into the equivalent of a low-grade nuclear weapon. Given that example, and how powerful Firesong is (and let's not forget what his ancestor Vanyel, managed while sane and lived to tell the tale) if Firesong hadn't gotten the help he so desperately needed, without a lifebond... what could Firesong have done in his self-destruction?
Similar to the above, on a lesser level — suppose someone of the right temperament were around and Firesong's need for stability forced them together. Once the immediate crisis passed, they would be stuck with each other, quite possibly not even in love (one character notes that it's usually love that cements the bond), an obstacle to Firesong's natural romance with Silverfox, and afraid to break the bond for fear of the consequences listed above.
Ho Yay: Both in-universe and among fans, naturally. In universe, certain areas are quite prejudiced against homosexuality and the suggestion of it must be avoided by those wishing to remain un-lynched.
Squick: Young Talia being forced to experience Companion Rolan's sexual encounters through their empathic bond. Also forced to experience her next-door neighbor's trysts with virtually every male Herald or Herald-Trainee.
Tear Jerker: In Arrow's Fall, Rolan arrives with the broken arrow — "all hope lost, do not attempt rescue" — a signal one hopes to never see. When Elspeth sees for the first time what has happened to Talia, she gets nauseated and horrified. The second time she sees: "I think she's dying!" She also feels guilty over her major dispute with Talia, just before Talia departed.
A tavern mistress explicitly hires Vanyel as her bard because he's gay, and so he won't steal away her serving girls. Then she gets mad at him when he refuses to sleep with her on request.
Heralds are bound to their Companions through an intense link that means they feel each other's pain and know the deepest details of each other's souls. It is an objective fact in-universe- and an oft-mentioned cornerstone of Valdemaran law- that this bond can only be forged between the most virtuous of souls. You'd Expect: For the cast to understand that a Herald cannot attack their Companion. Aside from the fact that that Herald would feel the blow as if they struck themselves, nobody with enough spite to do so would ever be Chosen. And Companions are perfectly capable of protecting themselves from such attacks- if not with their powerful telepathy, then with hooves. Instead: Everyone, no matter how knowledgeable or mature they are, assumes that Mags must have broken his Companion's legs. This while Mags is collapsed in a corner crying from the psychic pain.
What Could Have Been: According to the Ask Misty section on her official website, in 2005 Lackey intended the Collegium Chronicles to be set after the existing books, including saying readers would "get to see [Talia] as an exasperated mother".
Wish Fulfillment: Being Chosen and taken to Haven is a fantasy both in- and out of universe. Bardic Gifts, Mage Gifts, Healing Gifts, or just a bond creature would be nice too. Talia is one twice over, first as a teenage girl getting to be one of the heroes she read about and as an abuse victim getting to escape her situation and find a healthy and loving environment.
The Woobie: A lot of the major characters start out this way, Vanyel and Talia being the most Anvilicious of the lot. (Vanyel never stops being one.)