I see what you mean. It's sad how most teens are so desensitized to violence nowadays, and furthermore, how others who express their reservations about violence are labeled lame, oversensitive, delicate, emo, "gay," pussy, etc. (Heard this stuff thrown around all the time while at junior high school -_-)
However, I wouldn't be so quick to conclude that they are emotionally wounded and/or inured to pain (not necessarily saying you would, but). It may actually take a while before these kids truly understand the magnitude of what they see. From my impression of this video and a few others, they look like they're having lots of genuine, self-expressive fun, which I see as the wellspring of kindness and empathy. Therefore, I tend to believe their empathy, depth, and emotional sensitivity will blossom over time.
"It's way too dangerous to follow the advice of people who aren't empathetic, even if they are right."