I don't think he thought it through in any analytical way. There's a basic rule in dean's brain that says "Protect Sam", so he was going to stay with him. I don't think he'd thought through "will i wait for sam to kill himself and then I'll kill myself, or will i let Sam turn rabid and kill me..."

But death was in the air so to speak, and I think just the thought of things STOPPING - not having to worry about his damn promise to john - was a relief.

so really my answer is "all of the above"