Death in the movies

WARNING: This post is not for sensitive/squeamish readers

So last night the teen son invited me to watch Lucky Number Slevin and since my social calendar was amazingly free, I did. He also said it was a fine movie for his 14 and 12 year old sibs, so I let them watch too.
Well.
Let me first say, there's a lot of blood.
And shooting.
And dying.
And more than a little skin.
I kept yelling my son's name every time someone got knocked off or was doing something, erm, private.
"Sorry!" he'd say. "I forgot about that!" "That's the last time!" "Oh! Well, I mean THAT's the last time!"
It WAS a good plot-twisting story and (sadly) my younger kids thought nothing of the blood. However, at one point, someone (don't want to give too much away if you are going to watch it) is killed by a clear plastic bag over their head. Now THAT bothered them. And it bothered me as well. And I'm sure just the thought of it bothers you.
Why did that one incident affect them when 10 shooting deaths didn't?
I think because they could relate to that suffocating feeling - who hasn't had a blanket or pillow or something tossed over our heads/mouths accidentally (or by one of those type of friends who think they are being "funny") and had that panicky feeling?
Not many people (hopefully, thankfully) can relate to the blood-spattering gun play we see on TV every day. (And maybe it's partly BECAUSE we see it every day (on TV/movies) that it doesn't touch us as easily. )
And WHY, you may be wondering, am I writing about such a gruesome subject?
Definitely not because I'm going to write about people being killed in my middle grade novels!
But I'm interested in what moves children, what affects them. And I think one ingredient in that is how
well a child can relate to the main character's feelings - those universal truths that are the same for everyone. Basic experiences that we all, as humans, have all gone through. If we as writers keep those basic human experiences in mind as we write, we will created characters/stories that our readers will relate to on a gut, been-there type of level.
        The fact that my children were unaffected by all the shooting/bloodshed - well, that's a subject
for a different post all together. (rolls eyes and shuffles off...)