What does "Only for them, and mine eternal jewel Given to the common enemy of man, To make them kings, the seed of Banquo kings!" mean?

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.

In this soliloquy, Macbeth expresses his frustration due to the fact that he has killed King Duncan but is still jealous, envious, and suspicious of Banquo because of the witches' prophecy that Banquo's offspring will become kings of Scotland. In this metaphor, Macbeth is saying he has sold his soul (his eternal jewel) to the devil (the common enemy of man), but he cannot bring himself to say as much in plain words because he doesn't even want to think about that terrible truth. That is why the words "Given to the common enemy of man" seem to come out so hastily. Perhaps Shakespeare intended the actor to say, "and mine eternal jewel," then take a deep breath, release all the air from his lungs with a sigh, and then say, "Given to the common enemy of man" with his lungs virtually empty. The fact that the actor is saying the words "Given to the common enemy of man" with empty lungs naturally forces him to speak the eleven syllables quickly, and this is what suggests that Macbeth doesn't want to dwell on what lies ahead of him after his death. Since Macbeth feels he is already damned, and that therefore nothing worse can happen to him in the afterlife, this explains why he becomes more and more tyrannical until his ruthless and insupportable behavior forces numerous subjects to flee the land and ultimately brings an invasion of an army from England led by Malcolm.