No you don't. It makes perfect sense. M'Benga knew there were Klingons on the planet, he knew there was a chance he'd be captured, he knew he would probably not be able to smuggle a weapon, but he also knew he could take the serum and give himself a temporary hand-to-hand combat advantage based on his experience in the war.
It's bigger than that, but you have to connect some dots. When they first bring up the ion radiation, before they ever leave spacedock, M'Benga rattles off the cause and Chapel immediately jumps in with "We weren't in the war, he just reads weapons manuals." As though M'Benga shouldn't be talking about the knowledge he is sharing.
Chapel later asks M'Benga if he's alright, in what appears to be the context of dealing with Klingons. He responds something to the effect of "Can I ever be alright? But I'll manage."
Then, right before they take whatever the green (goblin/hulk) serum, Chapel asks M'Benga "Do you always carry it?" She has seen whatever this is before, knows what it does, and is only mildly surprised that M'Benga has it on his person.
The pretty clear implication is that these two got up to some dirty, probably violent stuff during the Klingon War that isn't supposed to be public knowledge. I get the impression that M'Benga was traumatized, either by something done to others that he witnessed, something he himself did, or both. Combine that with the anti-Klingon strength serum? M'Benga doesn't like Klingons. He actively carries a chemical to help him defend himself against them HAND TO HAND, which is the kind of thing that suggests, to me anyway, that it's personal.
It isn't OUT OF CHARACTER. It's ESTABLISHING character. Something happened to M'Benga and Chapel during the war, which will presumably come up again later. This is laying groundwork for later developments.