He's the second most power being in the universe... seeing that "creation" was an "abomination" - he revolted 3 seconds after creation was created and was defeated. Where he was cast down into hell and made "Satan" - an honorary title for the "governor" of hell. He's not the only Satan that hell has known... but he's by far the most powerful.
From the universe of the film "Constantine" based on the comic "Hellblazer" (I know that's the DC universe - but Hell is alive and well in the Marvel Universe too) - brilliantly portrayed by the talented Swedish actor Peter Stormare - Lou is a nasty guy... he's got serious underworld cred - tats and bad complexion and a tongue that likes to do creepy things on the faces of the souls he claims.
He is a "big picture" kind of guy. He isn't above doing the right thing to keep the balance... after all - this world is his... or so he says. He is especially happy about collecting souls of the damned. And Tony Stark is going to be a favorite play thing of his - I have a sinking suspicion...
I figure the Cap knows that evil exists and sometimes evil can be used to accomplish the greater good. And with Lucifer on the Captain's side... Team Cap can't loose.