They are space Marines how have cast there oath of loyilty to the emperor of mankind in order to gain the power of the Dark Gods of chaos. Heretic are the same thing it just that there random civialin that pray to the Dark Gods. I it highly likely that there will be traitor guardsmen, but how nows?
In Dark Millenium Online trailers you can easily see that there are
This info came from
and the Dark Millenium Online trailers
P.S. I'm the one how made Imperium of Man and Orks I just remebered my password