In Street Magic, it states that a twisted magician does not not abandon his mentor spirit, but that he focuses on the mentor spirit's darker aspect. For many mentors that grant purely mechanical diadvantages (i.e., -1 to Combat spells), this would mean no change. But some mentor spirits, such as Wise Warrior, Dragonslayer, and Dog, impose moral imperatives on their followers. So, for example, if a character who follows Wise Warrior becomes twisted, does his personal view of what constitutes a "dishonorable" action change? Or should the GM alter the disadvantage to something like "-1 to all actions for allowing a defeated enemy to live, until he atones for his actions"? What exactly is the dividing line between twisted and toxic?

To me, the line is that the twisted magician can be said to follow the same basic ethos of the mentor spirit, only twisted. Toxics pervert the original ethos of the mentor spirit into almost the opposite. A twisted dog shaman might be obsessively protective of his friends. A toxic dog shaman would be like a rabid dog, the opposite of man's best friend.

I'd go with the obsessive angle. For the Wise Warrior archetype think of those cheesy Darth Vader-esque villains who DO NOT put up with failure. A twisted Wise Warrior would probably hold things up to impossible standards and deal violently with those who feel to meet them.