A reason is a reason. You would try and kill him anyways even without an excuse.
Instead of targetting the person think of targetting his supplier (suggestion)
Also look in your own ranks before pointing fingers at people. I had several illeggal venoms used on me by Isamu but when I told some snakes (who just happened to be killing Shallamese without reason at the time), they ignored me.
Let he who is without guilt cast the first stone. In this case Guild, and yours is without guilt.

Penned by my hand on the 1st of Ero, in the year 246 AF.