Jon Irenicus.
An elf wizard prodigy that had it all, including the personal love of the queen. He also had an ambitious, weaker and more insignificant sister, whom Jon happened to love dearly (in a brotherly way).
Cue manipulations, awakened greed and ambition by the sister and they attempt godhood by killing the Tree Of Life and dooming the elven nation. A failed attempt, punishment by gods by ripping out their souls and making them human and exile later, they end up researching divinity and soul transferral.
They attempt to circumvent the curse, the sister Bodhi by applying vampirism. It backfires spectacularly (as she is now twicely cursed), forcing Jon to speed up his plans and horrifying experiments, even as he has lost the capacity to feel any emotion beyond hate despite longing for those missing feelings.
Cue Jon hearing about children of the dead god Bhaal, the protagonist (one such Bhaalspawn) getting captured, one of his party killed brutally, his kid sister forced to watch and the memories literally burned to her memory (among with various atrocities, permanently killing her innocence and somewhat her sanity) and painful experiments on the protagonist. All the while Jon keeps muttering about 'how much potential you have'.
That is when you escape, but the wizard retains hold of your deeply traumatized sister. All you know is a madman sorcerer/wizard has tortured you and yours for no discernable goal.
And then, even as your soul is ripped out and restored and you begin to fight the influence of your dead father (the God Of Murder), Jon and Bodhi with divine souls begin their revenge on the elves. All the while you begin know more and more of Jon's life so far, and his motivations. All culminating in the elven queen and Jon discussing before the (first part) of the final battle, to save the elves and return your soul before you waste away:
And you can't help but feel empathy for Jon. He might be a rabid dog needing to be put down, but there is very little joy in it.