Ok, here I go with my take on Itachi *rubs hands*
While what everyone else has said is great and all, I think we've missed one thing about Itachi's character as the slayer of the Uchiha clan. His mental condition.
Itachi was only 13 at the time when he carried out the massacre, when Itachi found out about the plans for the coup, he was mentally shocked. It's a lot and too much for a 13-year old to take. Especially when he loved the village. (I believe it was Minato who had a hand in this) Moving on, as he was so young, it would've been easy to confuse him and take advantage of his mental instability at the time.
I believe it was Madara and Danzo's doing here. They toyed with Itachi's mind so that he would ultimately carry out the massacre sparing no one even the innocents. In fact, they wouldn't even need Shisui's eye for this. Madara's mind games were more than enough to mess with Itachi. Itachi, as smart as he was (possibly on par with Shikamaru at the time) couldn't possibly think straight enough as his mind had yet to fully mature at such a young age. As for Shisui, he may have also been messed up by Danzo as well, but he realized what Danzo and Madara was truly aiming for. Alas, it was already too late when he realized it, as a result, he lost his left eye to Danzo. As a last resort, he passed his right eye onto Itachi.
I'm less sympatheic to this argument. Part of it is my profession. On a daily basis I meet people who have turned the removal or shifting of personal responsibility into an art form. I've had an endless list of excuses pitched to me about why nothing is their fault. It's something or someone else.
Anyway, my basic question would then be....if Itachi is less responsible because of his age, mental state, Danzou, and Madara's influence, doesn't Sasuke get to use this excuse as well? From his perspective, he is a six year old who watches his brother murder his parents and entire extended family in what would, at the time, appear to be cold blood for no obviously identifiable reason. So the family he's grown up with is taken away by the person he idolizes the most and he is left growing up alone. It's not clear who, if anyone, took care of him during the next six years.
Yes, the village didn't ostrasize him the way they did Naruto and treated him, at least in the academy, as a prodigy, but as far as anything outside the academy...what did they
do? They may have heaped praise on him, but was anyone genuinely there for him at any point prior to that? Naruto at least had Iruka toward the end and to some extend the Third. Sasuke then gets the cursed seal from Orochimaru which is supposed to have a negative influence of over him and draw him to Orchimaru. When he goes with the guy for three years who knows what he is told to him or whatever pyschological pressure is put on him.
He's then told a story of some truth, half-truths, and outright lies by Madara that includes the part about a brother he who was the source of the of his most traumatizing childhood event and, at least in his mind, is pushed to do it by a village that distrusted and wronged his clan. So, in short, the story told by Madara is one is which his clan is repeatedly unjustly oppressed by the Senju and it's decendants. Then when they try to do throw off that oppression, the oppressors use one of Uchiha's own to mercilessly slaughter them all. This is quiet a manipulation by Madara because the impression I am left with by Kishimoto is that the Uchiha, in particular Madara, repeatedly dug their own grave while the story Madara spins is quiet the opposite.
So...people toyed with Itachi. Well, they toyed with Sasuke too, but the sympathy felt for Sasuke, especially on this board, is much less and people are less willing to let Sasuke off with this excuse. If Itachi isn't morally culpable for this decision then Sasuke should at least, get some of the benefit of doubt here if we are going to use age and manipulation as a way to excuse the choice he made.
That said, I actually don't think Sasuke nor Itach should be able to use this as excuse to avoid personal responsibility for their choice. Itachi still chose to kill them all. Sasuke is chosing to pursue vengence on Konoha as punishment. Naruto's own plight illustrates that you can have a crappy past and still make the right decision. Sasuke is responsible for the choices he makes, but so is Itachi.
I think there are both reasons people seem so much more willing, some legit, some not. The important distiction is what they seek to do. Itachi wanted to preserve peace. And while peace itself is not an absolute good (Madara's plan would instill peace, would it not?), the type Itachi wanted is portayed as morally good, while Sasuke wants kill and destroy. And not just those involved, which, if I recall, is made fairly clear by Kishimoto. What they seek is different and because of that Itachi is seen as good and Sasuke seen as bad. Consequently, people are more willing to...at least look past what Itachi did while they can't do anything similar for Sasuke.
And this is where I completely agree with trixie about Kishimoto poorly developing all this. And the more I think about the stronger I feel about this. It's never explored about how pervasive the desire for rebellion is among the clan and who is involved. If there are other innocent children like Sasuke involved then isn't it horribly hypocritical of Itachi to off them, but leave Sasuke because it's
his brother. He's human, but Kishimoto ignores all of this.
It's so disappointing that Naruto now knows what Itachi did to him and is a chief reason for Sasuke's current course of action and doesn't question Itachi on the choice he made. Itachi warped his best friends mind and he has nothing to say upon learning the truth of it all? Second, as trixie points out, I don't think it's obvious that Itachi choice was the right one, but is anyone seriously questioning it at this point? Did anyone ever do that? Even Sasuke just shifts blame.
The other reasons I think have to do with double standards because so many people on this board hate Sasuke.
As I said, in my previous post I have some sympathy for Itachi, but that stems primarily from the fact that there was no morally clean choice available too him. Once he decided that the village was something worth protecting, it was chosing between a bunch of bad options regardless if anyone manipulated or pushed him.