Lets say GZ didn't have a gun and followed the exact same steps. He basically saw something suspicious, followed, and got into a confrontation. It wouldn't be as big a deal most likely.
I guess in my eyes, the gun issue comes into play at the moment he decided to use it. If he was indeed on his back, head busted, and getting pummeled then he did what he thought was necessary.
But if he went in gun drawn and looking to kill, that's a different story. If he thought that having the gun gave him increased leverage to go after someone, that too is just irresonsible.
I'm just glad we've graduated from the images of the sweet innocent 13 year old... one of those texts about getting popped in the chest was a bit eerie.