What if Sasuke is that way to all his enemies, and is really the good guy?
To Deidara: "Deidara! Can't you see that your partner is the mastermind of Akatsuki? He even has the Sharingan that you so despise! I must kill you to prevent him from controlling you and making you his puppet!"
To Itachi: "Itachi! We're both bad guys now! But I have your best interest at heart, so I must kill you and have others spread your good name so you will become a good guy again!"
To Killer Bee: "Killer Bee! The Akatsuki are after you and planning to use you as a Continental weapon! Here, let me capture/burn you so you may be taken to a safe place! I wore the Akatsuki outfit so no one would be suspicious!"
To Kages: "Kages! Tobi is going to start a war which will bring back a legendary figure that will destroy you all! Let me kill you now so his plans will be foiled!"
To Danzo: "Danzo! You killed a messenger frog and refused to help in the Invasion of Pain! I must kill you to prevent any more of your wrongdoings!"
To Kabuto: "Kabuto! Tobi is using you and Madara has broken free of his Edo Tensei binding! Since Itachi has ordered me not to kill you, I shall let him use the legendary Izanami to see the folly of your ways!"