As Ross and the Old Man are marveling at the fact that King Duncan’s horses ate one another, Macduff appears. Ross greets Macduff in most ordinary way, saying “How goes the world, sir, now?” Macduff’s reply is edgy: “Why, see you not? (2.4.21). In Macduff’s place we might say “What do you think?” or “Just take a look around you.” After all, a good king has just been murdered.
Ross then asks who did the murder. This is probably not an innocent question. Both Macduff and Ross heard Macbeth explain that he killed King Duncan’s grooms because they killed the King. Just the fact that Ross asks the question seems to show that he thinks that maybe Macbeth’s explanation doesn’t hold water. Macduff repeats the official line: King Duncan was killed by his grooms, who were bribed by Malcolm and Donalbain, whose guilt is shown by the fact that they ran away. Ross exclaims “‘Gainst nature still!” He adds an outburst against “Thriftless ambition, that wilt ravin up / Thine own life’s means!” (2.4.27-29). Ross means that what Malcolm and Donalbain are said to have done was not only unnatural, it was stupid, because in killing their father, they killed everything he could have given them. That is, if they did kill their father. If Macbeth killed him, then everything would make more sense. But if Ross and Macduff are thinking that Macbeth killed the King, they’re too cautious to say it out loud at the moment.
In any case, the Scottish nobles have already given Macbeth the throne. (This apparently happened in Macbeth’s castle soon after King Duncan was murdered. Once Malcolm and Donalbain had fled, Macbeth, as Duncan’s cousin, had the strongest claim.) Ross is on his way to Scone to see him crowned, but Macduff is going home to Fife. Bidding farewell to Ross, Macduff says, “Well, may you see things well done there: adieu! / Lest our old robes sit easier than our new!” (2.4.38). Macduff is wishing everyone well, but also suggesting that they may find that Duncan was a better king than Macbeth will be.