Macbeth makes his purpose in hiring the murderers clear to them; as king, he could simply order Banquo and Fleance to be killed, but they share certain friends and influential contacts, whom Macbeth needs on his side. Being the sort of king that orders seemingly frivolous executions, especially of those who were once close to him, is sure to turn suspicions and paranoia against him and make it harder for him to rule. Macbeth also cannot reveal that he needs Banquo and Fleance murdered because of a pagan prophecy, because this would surely mark him as an enemy of all. Instead, Macbeth lies that he hates Banquo, for specific reasons unstated, but this is enough to convince the murderers that he is on their side.
All of this suggests that Duncan's death did not mark an end to the conspiracies that Macbeth is obliged to contrive in order to keep his power. Macbeth is also more concerned than ever by the witches' prophecy, and is either unaware of, or ok with the fact that his actions may actually be fostering the prophecy rather than counteracting it. It also shows that Macbeth is now "above" murder; having done the deed himself, he might be seeing it as a lowly act, to be performed by servants, and this suggests that his lessening regard for human life renders it as a commodity rather than precious.