His desire of power was more important than anything else in his life. Once Lady Macbeth influenced him to kill King Duncan it ended up leading to him driving himself insane. Killing Banquo wasn't enough for him, either. He eventually went crazy by mid/end of the play. Once he got a taste of what it felt like to be in power, he didn't want to lose it. Killing off anyone who could take that away from him was his only choice (so he thought.)
