Ever been in a fight sinkspur? It's not exactly as neat and clean as "Rock'm Sock'm Robots."
If the police can demonstrate it wasn't self-defense, fine. But they'd better also be able to prove the guy wasn't kidnapped or being threatened with physical violence by the two other guys.
When the police take on the appearence of agressively defending two punks who got what they deserved, regardless of the character of the one who delivered it, they diminish the respect of their own badges. The way I see it, if they believe that the guy was kidnapped and threatened with violence, that ought to be enough to diminish their zeal to look much further into it.
If this was a drug deal gone sour (and there is at least reasonable doubt that it might have been), then a grand jury is the right place to sort this out.
Look, the world is better off without the two dead men. But, Deal may or may not have been in fear of his life. This could have been an execution.