I’ll have to rewatch that scene but I’ve watched the episode twice. It wasn’t my impression that he accidentally stabbed himself but that M’Benga was pushed to defend himself and stabbed him, thus finishing his mission on J’Gal.
I know some people feel that M’Benga killing Dak’Rah felt disingenuous to Trek but I actually liked seeing that even though these characters are diplomatic and controlled have their limits of humanity and right and wrong and that some atrocities truly are unforgivable. The characters aren’t perfect and make mistakes and I like that - I feel like it’s a more accurate representation of humanities struggle for peace.