Where the hell is butt sitter coming from? If the left lane was clear then you should have had plenty of room and time to safely pass three cars.

Otherwise the left lane wasn't safe to enter. You entered slower than the car behind you. You essentially cut him off. I don't blame him for passing you on the right and cutting you right back off. You damn near killed him.

The important corollary of "left lane is for passing only" is that you should never see a lit brake light in the left lane. If he's riding your ass, you forced him to use his brakes.