Which Harry Potter Villain Are You?

Power rarely comes without a price, and for Tom Riddle, that price was his humanity. Horcruxes are created through acts like murder – mortal sins that scar the doer’s soul and body, causing Tom to turn less and less human with every Horcrux he made. In all regards, Tom ceased to be, and Lord Voldemort came into being.
Imagine living forever only to have your nose fall off – you’d probably want to Avada Kedavra everyone, too.

Azkaban is likely the best reason. It’s a place catastrophically filled with Dementors that extinguish any fire of hope or joy in a person’s heart just by being near them. Bellatrix and the rest of the prisoners of Azkaban had been exposed to hundreds, perhaps thousands of them. She was slowly driven mad with constant despair and ennui.
Even before Azkaban, however, Bellatrix was already pretty eccentric, and enjoyed sadistically torturing prisoners. Azkaban only eroded her sense of humanity until only her madness remained.