Greg killing John's Dogs!
What did he think was going to happen? Did he think John wouldn't know who poisoned the dogs. He knew John was dangerous and bad news. That was really stupid on his part and an awful thing to do himself.
I would lose my crap too if someone killed my dogs. Of course he didn't deserve to be killed, but that was an extremely immoral and illegal thing to do.
This isn't to say John isn't a crazy physio path.