Cheating is reprehensible and this guy sounds terrible, but honestly I think you're even worse based on your short post. You joined this group knowing what it was, and are now power tripping and acting like you own this man's fate. This is not your place, and you're delusional if you think it's not about power. You have no relationship to this man and you're blackmailing him insisting that he'll have to prove to you that's he's changed his ways and is now loyal over an extended period of time... how is that NOT about power?

You should leave this man alone, and seek professional help. There are lots of people out there who do unethical, and terrible things but you're just a random 25 year old who is also doing an unethical and terrible thing. Stay out of it.