Itachi had an innate hunger to kill and destroy his clan because they limited him due to the responsibility bestowed upon him by their generally incompetent asses or he just didn't care enough about any of them and didn't mind killing them, this last part is probably more true than the first explanation. He probably saw them as a burden that tired him constantly. Itachi is also a psycopath, this means that he didn't see an issue with taking a human life so long as it "made sense" though, he only became attached to sasuke because sasuke was the pure-hearted innocence that carried the Uchiha blood, which is important to itachi's identity. Sasuke was the opposite of itachi, that's why itachi loved him. Sasuke was everything itachi wanted to be, and that is why he loved sasuke and asked for him to be spared. Almost as though itachi was asking for the tiny shred of innocence within himself to be spared as well.
