Dak'Rah kept egging M'Benga after he told Dak'Rah multiple times to back off and leave. He got into his personal space, and given how he knew what M'Benga did, it is quite believable that Dak'Rah was going to kill him... and considering the fight we saw earlier, he could do it without much difficulty since M'Benga wasn't juiced up at that time.
And even if it wasn't self-defense, M'Benga told him multiple times to leave. He instead pushed further and got in M'Benga's personal space, and considering Dak'Rah has proven he can kill him fairly easily (based on the fight earlier), every scenario puts M'Benga in the clear. Dak'Rah was a liar, a mass murderer, and a coward. Who knows how many other people he massacred in addition to that moon. He ordered civilians... children, to be killed. Dak'Rah had it coming.
Frankly, M'Benga gave him more warnings and was far more lenient with him than I would have been. I would have warned him only once and then stabbed that child murdering douchebag.
And this wasn't revenge. It was justice. He should never have been made an ambassador for the Federation. He should have been executed... or at the very least, serve a lifetime prison sentence.