AbdulAlhazred
Legend
Yeah, beyond that, I wouldn't consider a PC who does some questionable things necessarily EVIL outright. I mean, suppose I also commit some good acts? I could be neutral and not really concerned with good and evil all that much. Alignment is a swamp that will swallow up everything anyway. Presumably a bunch of highly pragmatic adventurers who regularly slay sentient monsters for their own monetary benefit are evil by any standard that would make you switch alignment in order to exploit a monster in some other way anyhow...You seem to have been playing under some serious house rules there. 1e D&D explicitly contemplated evil PCs (for example all assassin PCs). The rules for switching alignments imposed a loss of one level in 1e and then an xp penalty for a while, but that was for any alignment switch, including going from neutral to good.
Being turned into some kinds of monsters could turn you into an NPC, and there was discussion in 2e Ravenloft of turning those PCs who fell into dark lord path corruption into NPcs.