Why do the murders consider Banquo as their enemy? What did Banquo do to be their enemy?
In Act III Scene 1, Macbeth, having lately murdered the rightful king (Duncan), convinces two murderers (written "murtherers" in older texts) to kill Banquo *and* his son Fleance. He does this by convincing them that Banquo is the source of their problems, and poverty: "know / That it was he in the times past which held you / So under fortune...?" (75-8). He points out that all the details and arguments and proofs of this occurred at their last meeting (which occurred offstage).
The murderers are apparently convinced that Banquo is to blame for their poverty and low station in life, but Macbeth goes on to ask them if they are "so gospell'd, / To pray for this good man" (87-8). Sarcastically, he asks if they are such avid Christians that they'd pray for this "good" man "whose heavy hand hath bow'd you to the grave / And beggar'd yours for ever?" (87-90). That is, Banquo is responsible for all of their ills, both physical and otherwise, which have forced they and all of their progeny into poverty.
But just to make sure, he questions their manhood. When the first murderer says "We are men, my liege" (90)--by which he means both that they are not angels and thus do blame Banquo for their ills, but also that they are ready to avenge these wrongs--Macbeth responds with, "Ay, in the catalogue ye go for men" (91), suggesting that they have all the right physiological features, just like all sorts of mongrels meet the criteria to be dogs (92-4), but there is a rank to dogs; there are better and worse. Thus, he challenges his two chosen murderers to prove that they aren't "i' th' worst rank of manhood" (102).
You should enjoy the irony of Macbeth's falling back on the very same rhetoric that his wife used to turn him from "the milk of human kindness": questioning their manhood. :)