It tears him apart with every strike of his lightsaber, but he does it, because he's a Jedi and it's his duty and Obi-Wan never shrank from doing what was asked of him. In the following days, he just has to think of the burning Jedi temple and the slaughered youngling and he knows it was necessary.

At night, he has nightmares.

He sees his friends hacked in pieces, he sees the horrors of war, but worst of all, he sees the flames, the mutilated body of his best friend, the pleading eyes that were golden on Mustafar but sometimes turn blue in his dreams. He hears the screams - sometimes they're cries for help, sometimes there's pure hatred, but they're always, always full of agony and pain - and he walks away.

He left his best friend to burn and walked away.

He meditates, releasing his emotions into the Force. He reminds himself that he killed a Sith, a man who betrayed them and murdered everything they both held dear.

Anakin was already dead.