I dont think he killed the clan to get the mangekyou. He killed me to do that. He killed the clan for several reasons, in my opinion.

1. The rest of the clan had grown weak and complacent in there current abilities. He felt that they had shamed it because they never fully explored the limit of there bloodline, which is what he is doing now.

2. He wanted Sasuke to also gain the power he was talking about, and the best way to make yourself stronger is either to protect someone or hate someone with a passion. Itachi opted for the latter choice. Now whether he did it so Sasuke could eventually kill him, so that Itachi would have an opprotunity to fight someone on his level, or whether he just wants the best for his brother in some sick, twisted, Itachi's-thinking way... I dont know.