I have no idea how he is king of hell.. maybe hes been king for a while.. I do believe Azazel was before Dean killed him. according to demonology anyway Azazel was Lucifer's right hand man. but I really don't know where to start with Crowley.

Lucifer is considered Demons "God" as per that one demon (Casey.. in season 3) demons thought lucifer didn't even existed. So someone HAD to rule them.

I dunno. i really don't lol