Gang violence affects a family negatively because opposing gang members often target family members because they know that hurts the other person most and kind of puts them in control because the...
Gang violence affects your friends because if they live in the ghetto, there's drive-bys. Even if they aren't in the ghetto, when they go to school, they could have a friend who's in a gang....
Pretty much the same way that any violent act affects a community. Whether the act is committed with a gun or machete, it still disrupts the community, causes people to move, businesses to close, etc.