"Dissed" = Disrespected?
Yep! You got some young gangsta rollin' with his crew out to jack some drunk-ass white crackers and this dumb beeyatch with her faggy boyfriend disses him in front of his dawgs. If he didn't cap her they'd have taunted him as being weak and soft. She couldn't get away with this and now his homies can come visit him in the pen (that the ones who don't join him actually will visit is highly unlikely).
But hey, in the end he schooled that beeyatch good didn't he! He got to keep his street cred. He's an O.G. now boy!
And when he is a 45 year old man rotting away in prison with no hope for a future and having not felt the touch of a female in nearly thirty years he'll probably reflect on the fact that shooting some random woman might not have been such a great idea after all.