That is one of the issues. Why did he choose to be alone with a dangerous subject? Was he afraid the dangerous subject was going to get away if he waited for backup? Was he afraid that the dangerous subject would kill somebody else?
Both seem unlikely based on the knowledge Wilson had at the time he got out of the car and proceeded to chase Brown. If Brown chose to stand and fight Wilson was down to one option, if he was going to save himself, he would need to shoot Brown because Brown had already demonstrated at least equal physical capabilities and Wilson couldn't risk allowing Brown to get his gun.
Additionally, if he really believed that Brown and Johnson were such dangerous criminals then how could he have done something that would leave Johnson in a position to attack him from behind? Wilson may not be able to figure out what he might have done differently, but that goes a lot more to why Wilson was not a good cop then that there weren't other options.
It is very doubtful that Wilson's first interaction with Brown and Johnson went the way he claims it did in the face of the Arman video. Wilson had probably created a tense situation by an unnecessarily rude confrontation with Brown and Johnson who didn't react well to it. Then Wilson backs up and goes cowboy. Why didn't he wait for back up at that time? were the suspects running away?
And after his first cowboy move leads to a bad result he goes cowboy again and starts a dangerous single handed chase of the suspect that was probably going to end in his death or the death of a suspect that might be too excited to calm down sufficiently to submit to arrest from a guy he had plenty of reason to be afraid of.