Saint's Row 2 - Am I missing something?

Okay. I just bought this yesterday and it's the first I've ever played it. Fun game so far. But after I setup the church and take out hobos and other gang members, I go outside and see a few tags to spray - all of a sudden it's non-stop rape from the other gang (they're green, haven't learned names yet).

Am I missing something? I would kill a lot of them and the meter in the bottom left would fill up and display a skull - rinse, repeat.

What am I missing? I'm running out of ammo trying to keep up with killing these bastards.