Seems to me the knife was in the box on the table behind M'Benga's back when the altercation started. The view through the frosted glass did not show anyone picking up the knife. It is unclear who did.
And whomever grabbed it the end result is the same. Moral ambiguity over M'Benga having to kill Rah. If he struck first and murdered him there'd be a medical and psychological explanation for it. If he wrested the d'ktagh from Rah and then struck the death blow in self-defense then he has even more vindication. Either way M'Benga isn't even close to the more unethical character in that moment.