Crowley felt Castiel's presence. He must be down from Heaven. He knew that things weren't going well for him. Crowley would use that to his advantage. He could use anything to his advantage.
Crowley rolled his eyes. The angel was getting his jollies by watching Dean rake? Whatever.
He took him down to his new Hell. Made his proposition. Slowly reeled him in. It was always dangerous making bargains with those more powerful than you. But Crowley knew what he was doing. Castiel was an angel. He would never double cross him. It wasn't very honorable.