Why is Sasuke evil?

At first, he wanted power and didn't mind approaching someone as evil as Orochimaru to get it. He needed this power to avenge his clan, who were wiped out by his older brother, Itachi.

However, Tobi, under the guise of Madara Uchiha, explained to Sasuke that it was Danzo and the two Konoha counsellors, who gave Itachi that order, without permission from the Third Hokage. This has driven Sasuke to want to destroy Konoha itself also, blaming them for his clan's death too.