in the last D&D 3.5 setting we played, the party befriended a level 15 female angel, by the end of the setting the angel was level 20
in this new setting, my players are roleplaying as the souls of their previous characters now reincarnated into new bodies, new life and memories as well, but one of them recently remembered part of her past life and several fragmented details of it, among those was the good and happy memory of the female angel
now, the thing is, im planning them face the angel they befriended when they reach level 20, she is now a lawful evil infernal alied with an empire from overseas, the party is level 9, they will eventually cross the sea and find the now evil angel
BUT, as much as i have thought about it since the begining of the setting, i cant quite come up with a reason for her to fall to evil, the basic idea was that someone, maybe another pitfiend, tricked her into falling to evil, maybe showing her a situation in wich she lost hope and love for everything, but i cant come up with said situation
tl;dr - what reasons could an agel have to swich alignments to lawful evil?