Mostly I agree with your comments, my one caveat being that I'm not sure Hiro knows that Adam can be killed by chopping off his head a la Highlander. Everyone else and their mother knows, but Hiro doesn't as far as we saw. So maybe what he did was considered necessary by him? Eh, it's still really dark and bleak and I hope there are some characterization consequences from it.

Yeah, even if it had been the only way to stop him (as it may be in Hiro's mind), it's still a choice that Hiro had to make that will should affect who he is as a person.