Dr. M'Benga's morality seems basically ruined in the latest episode (perhaps understandably), it's quite intense. What's interesting is that he's still a member of the crew in TOS, serving under McCoy /Bones.
Spoilers:
Reading comments on Reddit, it seems as though lots of people relate to his version of justice. There's lots of fairly unsubtle hints that he murderers the Klingon diplomat/ex general, firstly the way he decides to open the box containing the murder weapon just before the incident, and his attitude towards justice that he explains to the captain, which is basically that he deserved to die because he was a lying war criminal. He also tells Dak'Rah to stay away from him, presumably because he believes that if he comes any closer, he may lose control and try to kill him. The weapon is conveniently ready for M'Benga to use against Dak'Rah, so I'd say it doesn't look good. The Dr. and Christine also try to make it look as though it is Dak'Rah's weapon, by making it appear as though the weapon belongs to the 'Butcher of J'Gal' - Their effort to cover up the truth is suspicious.
I guess we'll find out if he murdered him at some point, I find it interesting that lots watched this and thought that he probably didn't intend to murder him, but acted in self defence.
Another moment that is a little shocking is how quickly he decides to clear the transport buffers which contain a critically wounded soldier, to make room for some more - They are showing how damaged he has become by the war.