Important passage in Macbeth

In the scene right after the Murders killed Banquo, and Banquo's ghost came and "haunted" Macbeth, Macbeth said a really interesting and seemingly important line. Right after everyone left the dinner Macbeth was hosting, and he was acting crazy and scared by Banquo, Macbeth said, "...blood will have blood...". I thought about that for a while then I realized it could mean a ton of different things that make sense in the book. You can take it like this, "...Blood will have blood..." as in a person's family by blood will always remain in that blood passed down. That makes sense because he wants Banquo's son killed so he can have the throne in his family blood line. Macbeth doesn't want Banquo's family to end up being royal, he wants the royalty to stay in his blood line. The witches said that Banquo's sons were going to be kings, so that means the Royalty isn't going to end up being in Macbeths family.  Macbeth's only problem is that he doesn't have kids, so that won't happen until/unless he has kids. 

Another way to take it is "... blood will have blood..." as in now that Banquo's killed there are going to be more people dead for Macbeths pleasure as king. Macbeth is the top dog and he is guarding his position at the top. He will do anything to stay king. He realizes that throughout the book the only way to get his way is by killing people who are in the way of him being king. The only way to be king was to kill Duncan (the old king) and he did so. The only way to keep the royalty in his blood line was to kill Banquo's son, and Banquo so there would be no one telling about it (He killed banquo and failed to kill Fleance (Banquo's son)). Every corner that Macbeth turns he finds a threat to his position, and feels he has to take control of it by doing something about it. He never seems to let it be. I think that is important to the book because it foreshadows while displaying an important message. selfishness isn't the answer in Macbeth's case.