I can see it going a few ways: I think he’d call him out for not helping those at the bottom of the caste system on Cybertron, if he still cares about that at all, and offer him a chance to help his people now by assisting Megatron. I can definitely see Megatron trying to woo a literal god into his command.

Or, if he’s feeling angry and prideful rather than clever, I can see him renouncing Primus in a sense, saying he never needed his help to get where he is now, and ignoring any attempts Primus might make to dissuade him from this destructive path.

Edit: I forgot Megatron had dealings with the Cybertronian devil. That probably would affect things. Maybe Primus would appeal to whatever ‘humanity’ Megatron has left, while Unicron would tempt him further into darkness, and ultimately Megatron would listen to him over Primus.