
Nick isn't used to not getting his way. He expected Louis to be waiting for him and that he'd be unable to move on. I assume he was thinking that he'd get to have both the "right" relationship with the wife his father wanted for him and keep Louis on the side.
Plus, he wasn't expecting to lose Theo, who was a constant source of ego boost for him. Wouldn't be shocked if Nick tries to kill both of them.
"It was my first time!"
Cut to me, staring into the camera like I'm on The Office