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Actually, they're both off-screen shots. However, one death is particularly more mysterious than the other. If you drop Ben, and you get the 'Fall down into a hole to save Christa' death, than you most certainly never hear screaming or anything of the sort. However, the alley death is a bit more believable. Also, at the end, it appears that Kenny was 'lost' and not 'killed'.
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I haven't played them in a while, but I'm pretty sure that Kenny gets overrun either way.
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You never actually see Kenny die, he's just left in a very tough spot. If it were real life, it'd be safe to assume he's dead, but when it comes to film, games, or books, they're not dead until you see a body.