First off I think this is being blown way out of proportion by some.
Horford tried to show Pierce up and Paul got caught up in the action for a couple of seconds. I don't think that reflects anything on his maturity, intelligence or character. I've seen guys in pick up games go off on less than what Horford did. I don't think an immediate response is anything but human and that's it.
As for the whole gang sign thing, I think that's pretty far fetched. Pierce had better not have put up some kind of gang signal. Gangs don't take non members using their colors, signals, or anything else very lightly. The usual response is a bullet.
And I know what I speak of. I live in a gang infested area. I've seen an MS13 kid butcher another kid as just an innitiation and either a blood or a crip, not sure which, took a matchette to a kid in broad daylight in the main square of my city for the simple reason that he thought the other kid was wearng his gangs colors.
I don't even want to get into what has happen in my area because people tried to clean up the gang tags on the walls of their houses.