It IS a comedy -- Stewart's character shows up at the end alive, parasite-free, and with an eye-patch and missing fingers. Not only that, but he's smiling and waving at everybody. I'd say it was an exception the writer's made for his character, but there's others that get mangled and killed and show up at the end too.
I haven't seen this movie, and I gotta say I was really expecting Jon's character to attempt some misdirection before trying to kill our plucky teens.
"So, you think I'm an alien? You're absolutely right. I was born and raised in Ontario. I've filed all my paperwork with the INS on schedule and my citizenship is pending. (starts to tear up) I've done everything expected of me; what else do you want? I think you need to ask yourself why you hate immigrants so much. Now get out of my sight before you realize I'm the shortest one here."