I went with "He intended his life to end when Sam's did," BUT...I take it more in the sense, "OMG, Dad has asked to do something I don't think I can do, so, I'll just make sure Sam's infected, and THEN he'll try to infect me too, which is when I'll shoot him and then myself, because I can't take the chance I'll get infected and spread it as well, and this whole damn thing will make it so I have a decent reason to shoot my brother, and it's not as if I'm not obeying Dad, and OMG we're so screwed. I can't take it much more. Let's go out and that's it."

*G*
That is to say, I don't believe in Woobie!AT ALL COSTS!Can't live without Sam! Dean.
But I do believe in a young man who's lost his mother, father, has a grim perspective in front of him of his brother turning evil...the infection is a way out for someone on the brink of a breakdown.