If Macbeth not kill the king? he can be a king?

If Macbeth didn't kill the king, we would never be the king. He would just be Thane of Cawdor. When he killed King Duncan, Duncan's sons (Malcolm and Donalbain) thought they were next, and were frightened. Donalbain fled to Ireland and Malcolm to England. In the first palce, Macbeth was a great warrior and the three witches predicted he would be king. He was the only decent choice around, so Scotland picked him as their ruler.