I believe M'Benga that it was self defense. If you think about how Dak'Rah was goading him it was obvious that he wanted to suss out what our good doctor knew about what happened in J'Gal. Remember, as M'Benga was killing his 2nd, 3rd, and 4th in command he was fleeing and knew there was someone out there who knows the truth of what happened. Given the large number of casualties, he most likely is on high alert when he comes across anyone who "was there". He figures Ortegas and Chapel don't have it in them to assassinate 3 Klingons so that leaves M'Benga who (upon rewatch) has this dark edgy energy, so... maybe? When he finally gets the truth out of M'Benga he has no choice but to kill the person who can disrupt his new cushy life.

Thinking about it, either M'Benga dies or he dies. Let's say M'Benga spares his life and exposes him for the coward that he is. He tried to kill civilians! His own people too! The Federation could never allow him to continue his cushy life and his fellow Klingons would never allow him to live after finding out. To not make that attempt to kill M'Benga first would be an illogical choice.

Also other questions to consider:

Did M'Benga hold back during the sparring so as not to give away his abilities?

How threatening was Rah? What if Rah had just said something like "I'm going to make sure no one knows the real story" and M'Benga takes that to mean Rah will kill him at the first justifiable opportunity? So might as well defend himself now?

M'Benga would have to know exactly where to stab so that the 2nd Klingon heart doesn't keep him alive. Otherwise what are the chances of hitting that right spot by luck?