Dean didn't really mind being a demon. In fact, he found a certain purity to it. Much like his time in Purgatory, being a demon was freeing. It was like an emancipation from the pesky emotions that all humans were shackled to. He did feel a pang of regret at having to kill Sammy, but it was his destiny after all. Cain had pointed it out to him, and it had taken reverting once more to the demonic state to release him from the hold of his familial bonds. He was finally free of Sam's whining, complaining, and desperately seeking a way to remove the mark.
Charlie dying for him had really been the last straw. The guilt had eaten at him, tearing him apart from the inside. It wasn't really Sam's betrayal, but yet another person having died to save him. He was tired, and he hated his life. Hunting wasn't enough anymore; he needed something pure and now he'd found it. His next step would have to be Castiel. After all, Cas had promised to put him down should he turn again, and he didn't want to die. Not like that.
