Explain what Macbeth means when he says, "Things bad begun make strong themselves by ill" (III.ii.56) in Macbeth.
This quote is actually the final line of a speech in which Macbeth delivers something of his own incantation, similar to Lady Macbeth's speech in Act One, scene five ("Come you spirits/That tend on mortal thoughts...). He calls not on spirits, but the night itself to use its "bloody and invisible hand" to assist the "deed of dreadful note," which is the murder of Banquo and Fleance.
The fact that he is not just being metaphoric, but actually summoning evil is supported by line 53:
Whiles night's black agents to their preys do rouse.
He concludes with the quote you have given, a statement which insinuates that now that he has summoned the "black agents" of night, that they will take matters into their own hands, and the outcome, whatever chaos this might involve, is up the actions of these forces. And thus the deeds themselves which are "bad," add unto themselves, without further aid from Macbeth.
When Macbeth says that "Things bad begun make strong themselves by ill" he suggests that prior bad acts beget even worse acts when prompted by an evil nature. Macbeth has already begun his string of bad acts with the murder of King Duncan. Now, he plans to have Banquo and Fleance murdered simply because he thinks they pose a threat to his position. By this point in the play, Macbeth has succumbed to his evil nature, so his bad acts will not stop and will likely grow worse. The line is actually an element of foreshadowing--after this murder, Macbeth does plan and execute other murders. The truth in this statement does suggest a chaotic world because there is no point when the violence ceases. At the end of the play, Macbeth realizes that he is wrong and that no good has come from his bad acts, but even then he does not stop or repent--he goes on to battle Macduff, and Macbeth's tyranny finally ends when he is killed.