"Why not you?" Crowley asked. They had history, for one. The angel versus demon thing was another factor. "Maybe I just like your company," He shrugged. That could have been as true as any other reason.
So he was a demon. He played games. he sometimes did spiteful things, like keep her Grace from her when she sort of needed it. But he'd only done it to make her stronger. he hadn't, and he wouldn't have, let her die. And he'd needed the Grace to try and smooth out the rough edges off Sam's damaged soul. He was a bastard by nature, but sometimes he did things with a dual purpose; sometimes he actually tried to do something nice, or at least halfway decent.