How exactly do you know he isn't working directly for Russian intelligence?
But it's much more unlikely that he'd be an actual Russian intelligence employee since they're usually Russian I think.
The more simple answer is that he's just being run by them through money, blackmail, extortion, or some other leverage. Why employ someone when you can just get them to do what you want with simple threats?
His hatred towards the US goes back a very long time and has certainly only grown stronger given how he’s been treated.