3.

Dean killed Ruby six months after he returned from hell. As it turned out, she was the reason Sam was what he was. She'd introduced him to his demon side and slowly pushed him deeper and deeper until it had consumed him. Dean had yelled at Sam for hours when he found out. It wasn't because Ruby was a demon, or what Sam had let her do, but because she wasn't Dean.

Sam belonged to Dean, no one else.

So Dean waited, and the next time she came sneaking around, her stuck her own knife in her gut. Sam shrugged when Dean told him what he'd done. She wasn't important, not now that he had Dean back. After the spell, after they left Bobby's, they hit the road and wandered for weeks with no destination in mind. There was some blood in Springfield, some carnage in Nashville, and then they ran into Meg in Cincinnati.

After "hello" began to wind down, destroying most of a city street in the process, she spent the next couple of hours gloating. Dean knocked her through a brick wall for making a comment about John and she returned the favor over a snide remark about Yellow Eyes. When they finally sat down for a couple of drinks, Meg made the mistake of coming on to Dean. Sam calmly stood up, took out Ruby's knife, and slit her throat.

Dean belonged to Sam, no one else.