Death in general isn't considered a reason to have a high rating. For instance, there's death in G-rated Disney movies. (One of my favorite Disney movies has two deaths in it and it's rated G.) The difference is how the death happens. Like with the Disney movies, the death happens off-screen. No one really sees how it happens. The viewers just know it happens in some way. If the death is completely described, with blood flying everywhere and pure violence, then it would get a higher rating.
Besides, if you see the death as appropriate for teenagers, then that doesn't reach the M rating.