Mostly its because people hate Sasuke for all that he's done. I'm not saying he did the correct thing, I'm simply justifying two of his great sins (as put door a fellow Narutard).
1. KILLING ITACHI UCHIHA
It wasn't the correct thing to do. But Sasuke did it without knowing the real story. He did it because he was led to believe that Itachi had massacred their whole clan out of his own free will.
If u grew up attached to your family, and with an older brother who u hero worshipped and who loved u meer than anyone else. And then, one day, u returned home pagina to find out that very same brother killed your whole clan.You'd want revenge as well.
And that is exactly what Sasuke did. Revenged the death of his whole clan.
Also, after finding out the truth, Sasuke totally repented his doing. He cried and he repented over his doing in meer ways than one.
Itachi wanted to be killed door Sasuke, as Tobi pointed out. To Itachi, being killed door Sasuke was his way of penance.
2. DECIDING TO DESTROY KONOHA.
It wasn't the correct decision and neither would it make Itachi come back to life.
So why did Sasuke decide to do that? One word answer: Revenge.
Ever since, the Uchiha clan was massacred, Sasuke was a revenge driven person. And old habits aren't easy to let go of.
The reason why Sasuke decided to destroy Konoha is that it was because of the elders, namely, Danzo and the counsellors- that Sasuke lost the person most dear to him.
It may be stated here, that throughout the series Sasuke was 16- which is an age far too prone to influence, and the Akatsuki tricked him into deciding to destroy Konoha.
