Dean wasn't thinking it, not really. He diverted around thinking it and was way past the point where he could. Dean honestly didn't know if what he slipped John would kill him, he was just sure it would knock him out and he... he wasn't gonna think about it. If it happened, it happened. Dean's focus was pretty damn narrow.

Honestly, I don't think he was putting very high odds on John's survival with two werewolves loose in the room either...