Actually it was fairly obvious to anyone who has seen the preview on T.V. or while waiting for another movie. He believes he is the only person that survived, with a little hope that someone might be around. But for the most part he was completely alone except for sam.

So what usually happens to someone who is alone with one companion? Safe to guess that Sam would bite the bullet.

Also in the preview Will is shouting, the actual scene that came from was funny yet tragic.

Then you see him kinda carrying something, would have to see it again but I think it looked like a dog... Dogs are not to common in the infested city of New York.

Anyways... tragic and it tears your heart out... but easilly to see what was going to happen.

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[original looks better, but the site wont allow its awsomeness, so i had to shrink it a bit]