Macbeth says this after he finds out that Fleance has escaped. Fleance will be a problem in the future, since he will have children who will become kings, but for now Macbeth can let him go and deal with other things because Fleance is of no immediate threat to him personally. read more
Literally, with NO sarcasm, he thinks Macbeth is the "good guy," Fleance killed Banquo, and Duncan's sons killed the king. Sarcastically, he seems to be saying the exact opposite: It is ridiculous to think Duncan's sons would kill him, Fleance had no reason to kill Banquo, and Macbeth has made a mess of Scotland. read more
It’s really important to remember that the one Macbeth wanted dead was Fleance, Banquo’s son. Banquo because the witches had foretold Bamquo begetting kings while Macbeth would not. With Fleance dead, no heir to Banquo and no line of kings. Banquo had to die to make sure he did not father any more potential heirs. read more