They can, in fact, cast without any limbs whatsoever. What's more, the speed at which they cast spells don't seem to have anything to do with their AGI. So leprosy does absolutely nothing to make mages less threatening.
I just tried it in Wiz mode. I imagine there is a killed_petrus=true flag in the code somewhere needed to trigger chaos win. I walked out of the city with his left nut and nothing happened. Maybe its just wizmode?
WM triggers endings as normal, so you'd be right. Killing him is the condition.