He is excited at the prospect of becoming king but also fearful about what role he will have to play in making it happen.
Explanation:
At first, it doesn't seem that he cared much about Banquo having heard the prophecy. Banquo was his best friend after all, but one he realized that he would actually be a king, he became so happy that he forgot the fact that he had to kill the current king. Once he thought about it, he was quite scared.
