What it is is ironic.
He's spent his own life saving people; bring the dead justice, keeping harm away from the innocent.
And now?
He buries bodies of entire families in the dark of night, he burns down homes to conceal evidences. He's brought entire neighborhoods to the ground to divert suspicion.
He follows his brother from state to state, does what he can to limit the deaths but his brother needs a certain amount. Dean gets the feeling it's like a quota, he needs a certain number of kills or else. So because Dean's Dean, and he's never said no to his brother and because he's faced with a choice, watch Sam kill or watch him die, he lets him kill.
He wasn't able to save Sam like he promised. Instead Sam saved him, by offering the devil himself his soul, if Dean got to live. So it happened, Dean got his soul back, and Sammy's became darker then the pits of hell. At lest they're together.
Dean wonders if Sam knows he'd rather die then see him turn into this shell of what he once was. Dean wonders if Sam knows that every life he takes, every human he kills mercilessly, he feel it like it's his own death.
But Dean loves his brother, and even though the logical part of him knows he's more demon then man, when Sam curls into bed and buries his head into his neck and sighs, just like he did before, Dean can't help but feel his brother's back with him.
Yet in the end, despite his love for his brother, despite his desire to always be with him, Dean can't handle it. So when Sam's in the bathroom he takes a gun from his bag and easily decides this is better then his silent death. He lifts it easily to his mouth and pulls the trigger, it's better this way, his Sammy is gone anyways.