Wow.
Where does violence like this come from, or I suppose more to the point, how do we remove it from society? Initially, I figured we as a species need to promote stronger family ties so that people don't feel the need to join gangs, but then I remembered family feuds and such. I suppose everyone needs to be able to learn how to forgive each other (that doesn't mean forgetting they've wronged you in the past... you can still not trust them, but you should decide not to escalate the violence.)
Calm down. It's 500 people in a city with 20 million. Have some perspective.