Let's just quickly recap. All we know FOR SURE about Itachi's past, not counting whatever 220's words are, is that a) Sasuke said he had unbearably good Shuriken skills. b) He showed his brother his 'murder' of the Uchiha clan, which could easily have been a fabrication, using the mangekyou sharingan. (After all, I somehow doubt that at any time Kakashi was stabbed by 70,000 Katanas, or even he would not have lived through it, but the mangekyou sharingan made him live through that.) c) He always tells Sasuke that he's too weak, and he doesn't hate enough, that he should get stronger, which started that very night.
d) He was in the room RIGHT after his parents died for sure.

Given these facts, I think it would be significantly less intelligent to make Itachi the murderer, when you consider that the last thing you'd want to do, being the murderer of your entire clan, is leave your brother alive, and tell him to get strong enough to kill you. I guarantee you there are plenty of people who can kill Itachi, if he wanted to commit suicide or test his limits, they would all do very nicely. We also know that at several points he could have killed Kurenai, but didn't. The same goes for Sasuke. If Itachi was a cold blooded murderer, these things would make very little sense at all. However, if on the other hand, he arrived after the murder had taken place and/or failed to stop it somehow, it would make a LOT of sense to keep Sasuke's focus on him, rather than having sasuke look for the real killer, and at the same time, if Sasuke DID get strong enough to beat Itachi, he'd be strong enough to defend himself from the killer. There really is a lot more going for the 'not killer' idea than there is for the 'killer' idea. And besides, don't animes have enough bad ass evil characters that only fight to test their limits already? You'd all really begrudge us one extremely badass and enigmatic good guy? Seriously. Get over it.