There could be a couple reasons as to why Macbeth killed Duncan before he had Banquo killed in Shakespeare's tragic play Macbeth.
1. After receiving the prophecies from the witches, Macbeth did not have a lot of time to really consider his actions. Instead of thinking everything out completely, Macbeth came to the conclusion that the crown was the most important thing to him. Fueled by both his wife's chiding and his ambition, Macbeth desired the crown above all else. Banquo was of no concern at this point to Macbeth.
2. Macbeth did not consider Banquo a threat until after the prophecies came true. Banquo was with Macbeth when the witches told him that he would become king. Therefore, Banquo was the only one who knew of the prophecies and that Macbeth's gaining of the crown may not be just "good luck." After gaining the crown, Banquo became a threat to Macbeth.
3. Macbeth was not really after Banquo as much as he was his son, Fleance. Macbeth told the hired murderers to kill both Banquo and Fleance. Banquo was really a bonus for him. When Fleance escaped both Macbeth and the murderers realized that the one they truly needed dead had escaped with his life.
We have lost
Best half of our affair. (III,iv--Second Murderer)
Then comes my fit again: I had else been perfect,
Whole as the marble, founded as the rock,
As broad and general as the casing air:
But now I am cabin'd, cribb'd, confined, bound in
To saucy doubts and fears. (III,iv--Macbeth)
In each of the quotes, both the murderers and Macbeth admit that the one they both truly needed dead, Fleance, had escaped.