Well, to be frank I would say 'Sorry, but no.' 

Playing as one of the antagonists would be a little different. That person would mostly likely be a co-GM and someone I would necessarily have to have some trust and knowledge of as a player. As what they do will directly impact the PC's efforts. Also if I bend for one person I have to bend for others. The rp is about a team of space marine veterans working together to cleanse chaos worshippers. Not to disparage or insult you in any way, I just haven't played many games alongside you. So I don't know you very well as a player.

If that answer doesn't satisfy you than all I can say is, i'm sorry but I am standing firm on this. I thank you for your interest and you are welcome to stay and make a loyalist character, but not a chaos one. 

If this idea doesn't pull through or when this one runs it's course I will be putting up an actual chaos themed roleplay that I have been working on alongside this idea.