In reading Shakespeare's Macbeth, I believe that the story is about different reasons for killing: distinguishing between honorable and dishonorable violence, but I do not see it as a plea for peace and human harmony.
There are a couple of reasons for my answer. First, this story was written for James I of England (formerly James VI of Scotland) by Shakespeare, to honor one of James' ancestors, thereby honoring James I. The motivation in writing it had nothing to do with a plea for peace. Shakespeare was a playwright whose professional successes depended primarily on the financial and artistic support of the King. Shakespeare was not interested in politics: he was telling a story—honoring the King's ancestor, Banquo. (Banquo is murdered but his son, Fleance, escapes.)
The English (and all of Europe) at this time are not unfamiliar with war. War seems to go on forever; and even though England is presently at peace, after James I's reign, war will return relatively quickly. However, at the time, there is no need to make a statement regarding peace and "human harmony."
As Shakespeare does with many of his plays, the facts have been rearranged somewhat for the sake of the art. Historically, Macbeth did not actually murder Duncan (who wasn't really such a nice man), but beat him in a revolt of Duncan's lords (against the King). Macbeth ruled for seventeen years accomplishing many good things as King, as opposed to a short time portrayed in the play. Malcolm did come to the throne after killing Mac Beth (as he was called then) in battle. However, within the context of the play as Shakespeare wrote it, it can be argued that the play demonstrates that killing is justified under certain circumstances.
[Shakespeare's] plays reflect not only timeless conflicts and resolutions, but a view of the Elizabethan society.
Macbeth is a murderer, guilty of killing a king. This was a mortal sin in the eyes of the Elizabethan audience. They believed that God chose England's sovereign, and that as long as Macbeth ruled, there was a disruption to the order of God's universe. This disorder would not be corrected until Macbeth was removed from the throne. Macbeth traffics with witches, another [religiously] offensive action to the Elizabethans.
Macbeth is also guilty of other deaths, orchestrated to protect his place on the throne. His kills his best friend Banquo and the entire family of Macduff because the thane refuses to support the King. Macbeth is a tyrant who should be punished in the eyes of the audience. They would have seen the deaths of Duncan, Banquo and the Macduff family has dishonorable violence on the part of Macbeth, and the attack of Malcolm and Siward against Macbeth as honorable violence.