To answer this question you need to look at Macbeth's soliloquy in this scene that he delivers after Banquo has left to go on his horse ride. If we examine this closely, we can see Macbeth's intense frustration about being king but at the same time recognising that the prophecies of the witches indicated that it would be Banquo's heirs that would ultimately be king. Macbeth thus reflects that it is "nothing" to be a king if he is not "safely thus," and has his dynasty secured. Note the way that Macbeth refers back to these prophecies in the following quote:
They hail'd him father to a line of kings:
Upon my head they placed a fruitless crown,
And put a barren sceptre in my gripe,
Thence to be wrench'd with an unlineal hand,
No son of mine succeeding.
The imagery used seems to emphasise the position that Macbeth finds himself in, as he has a "fruitless crown" and a "barren sceptre," as he transforms the traditional symbols of power and kingship into signs of his lack of an heir and a future. In addition, Macbeth is preoccupied by Banquo's nobility of character. He recognises that Banquo has a "royalty of nature" and a "dauntless temper of his mind." Banquo therefore concerns Macbeth because of the goodness of his character. He therefore decides to act, having both Banquo and his son killed.