There is a time when satisfying the people around you is no longer enough. There is a time, if you are strong enough and don't care about people enough, when you realize that you are stronger than everyone around you and you think you're better than them. There is a time when you need something more because you've always been the best, and there has to be more to life after you've reached the top at thirteen.
There is a time, for Uchiha Itachi, when he decides he needs to acquire the ultimate ability of the Uchiha clan no matter the cost. And there is a time when he needs to test it, to see just what being the best makes you capable of. He has never been challenged, and his only regret afterwards is that even the great Uchiha clan could not stand up to him.
He is not compassionate. He doesn't love Sasuke, because his brother is inferior, soft and always hungry for praise, companionship, acceptance. He is only curious, interested to see if this will make Sasuke stronger or destroy him. Because he doesn't think in terms of emotions, he doesn't realize that it will do both, make him grow strong and hard and dangerously angry. Because he is so much stronger than everyone around him, he doesn't think that Sasuke might one day threaten him.
Itachi doesn't care about politics, about which country is allied with which and who is plotting against whom. He doesn't care about having power over people, only about the power itself. He only cares for power because he is nothing if he stops being the best. He is a legend, and he has to live up to it, so he will join whatever organization and fight whatever enemy, but he will never care.