Gotta love the logic of "omg, he knew I was gonna do something bad to him so he did something bad to me? how DARE he!?"
Then again, it's probably oversimplifying things.
Question. If you had a great memory, and found out you soon would be transported 15 years into the past (no smartphones, have fun), would you A. Read a bunch of famous books so you can rewrite them yourself for fame and fortune, or B. Invest in Google, Apple, SpaceX, Netflix, etc. and then with...
No need to kill him. Let him live with the chaos inside his body and being unable to use his sorcery. No mage will help him, so he may live the rest of his life like that.