As Fate has a habit of coming around with or without intervention on the part of those concerned, if Macbeth hadn't killed King Duncan, something else would have and likely his sons as well, giving the throne to Macbeth in the process. read more
2) You assume that Macbeth only became king because the witches prophesy urged him to kill Duncan. He successfully convinces his wife that they will "proceed no more in this business." Duncan dies a few years later, and Malcolm becomes King. read more
(Or you can argue that it was fated that Macbeth would kill Duncan and that's what the witches were predicting when they said he's become king.) Lady Macbeth also encourages Macbeth to kill Duncan. She is almost most status and power hungry than he is. read more