they would probably not be justified,as they are in the commision of a crime and not in a place they are legally allowed to be,plus they're weapons are likely illegal.
remember to check your local and state laws,and remember these types of things go on a case by case basis!
Standing on the guys lawn with 15 of his 'homies' the gunman would probably look like a thug and talking smack. The homeowner fires a warning shot that the whole neighborhood hears and one of the gangbangers pulls out a gun and kills him.
Two weeks later the gangbanger is standing in front of a judge wearing his sunday best, hair combed, and a cross dangling around his neck like a choir boy. The defense paints a picture of him and a few of his chums minding their own business on this man's lawn when suddenly the 'crazed gun owner' opens fire and forces the boy- who was in fear of his life- to defend himself with lethal force. A parade of neighbors who heard the homeowner fire first, plus the 'testimony' of the other 14 boys who lie through their teeth will be admitted into evidence.
At the end of the day the homeowner is still dead and there is a better than average chance that the kid who killed him will get a slap on the wrist for trespassing, and possibly a firearms violation is the gun is not legally registered.
That's the way I imagine it would go down. IANAL, but that seems just messed up enough to be right.