Omar was upset with how they tortured and mitigated Brandon and left him on display.
The entire Omar character was created as a device to walk both sides of the line, and show two sides of the coin.
People think Omar followed the street code, but he didn’t. He followed his own code. His own code allowed him to feel more honorable (and appear more honorable to us viewers) than his peers in the game.
His own code allowed him to justify robbing or killing, as long as it wasn’t on bystanders. It allowed him to cooperate with the police to enact personal revenge. Etc.
Omar was his own entity, and didn’t bind himself to the rules of the game. It also ties in with the Greek mythology aspects of the show. He was bigger than the street, despite working in it.