At this point in the play, Macbeth has achieved his ambitious dream of becoming king of Scotland. However, even though he anticipated the price of killing Duncan would be high, gaining the throne has cost him more than he thought. He is beset with worry that enemies will undo him and former friends betray him. He cannot rest easy on the throne.
Therefore, as he thinks back on the witches' prophecy that Banquo's descendants will inherit Scotland's throne, he becomes bitter. He says "only for them," meaning by "them" Banquo's heirs, contemptuously, now realizing it was hardly worth it to sacrifice his "eternal jewel"—his salvation in heaven—so that Banquo's line could be kings. Since being king in the present moment is no fun and brings Macbeth no fulfillment, he seethes with anger that his work may "make . . . the seed of Banquo kings!" We can almost feel him gnashing his teeth and wanting to lash out at someone in his pain.
It seems clear from this passage that what motivates Macbeth to have Banquo murdered goes beyond merely getting rid of a threat. Macbeth actively wants Banquo dead because he resents the idea that his heirs might be kings: he wants to prevent that from happening out of spite. This shows the deterioration of Macbeth from a moral, if ambitious man, to a soulless, malevolent monster. He has fully turned on his former friend.