Using black magic or having it used on them? As in what really happened to Justin? He had the rep of being a very good Warden when he retired, but then he went warlock.. He's been called that what he did to Elaine and tried to do to Harry were clearly warlock moves, so what corrupted him? Was it just a series of wrong choices and wrong answers?
Well, I am not sure having Black magic used on you corrupts the recipient. I haven't seen anything to suggest that...although that would be very Star Wars and Jim is a big fan.
Justin's decent is a great mystery. But the big indicator is that his master was Simon Pietrovich, a wizard who broke the rules around interfering in mortal politics and was warned by his long time friend Ebenezar to stop or be taken out. Simon found the restrictions of the Council to be frustrating. This is all information from the Paranet Papers. I'd say that Justin agreed with his old master.
Justin did gain Bob when Bob was Kemmler's horrific demon servant. I imagine part of the corruption was from there. Perhaps he was warlock by then, perhaps not. Justin clearly knew about the Cycle and whatever the starborn's precipitate. He was planning to be a player in the Cycle it seems, but seemingly was foiled. It wouldn't surprise me though if Justin became more desperate and kept justifying using greyer and greyer magic until he forgot where the line was and was full on using black magic. Seems to be how it goes. Slippery slope and all that.
