I would kill him, even if my contract was over. I would have left him alone if he would have just tried to kill me, no harm there, an eye for an eye , and so on. But torturing, now that is crossing the line. It was just my job to kill him, why punish me ? Anyways, most likely I would send him and like 11 more people (who i hate and want to kill, also) invites to an private island that i got during my mercenary years. Saying that I'm a long-lost relative or something like that. Promising something good, they all come there. However, when they all come, i will disguise myself as one of them, and start killing them one by one. In the end, it'll be just me and good old Ryan. He dies, i laugh and run away.
Also, i make it look like Ryan did all the killing and then shot himself in the chest 4 times, sliced his throat, and pushed himself down an elevator shaft.
Flawless victory, eh?