[Xiahou Dun]

Are you sure about that?

Space marines are a pretty prideful (is that a word?) bunch. I imagine at least some of them would have had too much pride in their history and origins to just give it all up. Perhaps some of the captains, or some of the people who were close to horus?

But then, even if there were a few I suppose they're probably dead by now. At least there probably wouldn't be enough to make up a company, or even a squad.