[Lenas]

Well.. a half-explanation would be that Sasuke had zero knowledge of what the sharingan could really do. He wasn't a threat because he didn't know about all his potential. Not until he saw Itachi again. Konoha was under the impression that Itachi was evil because only 4 people knew the truth. I don't think the 3rd ever spoke about Itachi, and Tsunade doesn't know about it.

But to say that Sasuke wasn't a threat because of ignorance of his own abilities, would also mean that no other Uchiha child was a threat - so why kill them too? Perhaps because if every one of them lived, they'd be angry and dangerous as a whole?

I am a big fan of backstory. In hind sight, I'm not really upset anymore about Itachi being a good guy. People have been guessing that's the case for years.