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, most often when they tried to call up fire. 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 to see if a marriage could be set up between Ancar and Princess Elspeth) 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.
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!
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.
It's clear that Firesong is deliberately trying to invoke this trope: one example is that he has a firebird because he bred/created it himself in order to show off. He also takes to the role of a showman very happily, and it's when his loved one starts to ignore him that he starts showing cracks. There are implications that he feels that he has to be special because he's descended from Vanyel and has to live up to that legacy & that his mother was very strict and demanding. It's need to be loved or for the mental stabilizing effect of a lifebond that nearly sends him over the Moral Event Horizon.
Talia, besides being (as noted below) The Woobie, is an arguable Sympathetic Sue: she's a misfit in her community, an abused child about to be forced at thirteen into an arranged marriage with a man old enough to be her grandfather; and when she runs away, she's rescued not only be a Companion, but by the Monarch's Own Herald's Companion.
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 any 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.
What an Idiot: A tavern mistress explicitly hires Vanyel as the tavern 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.
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.
The Woobie: A lot of the major characters, especially early on, start out this way. Vanyel and Talia are the most Anvilicious of the lot. (Vanyel never stops being one.)