Perhaps the most important insight into humanity provided by Macbeth is its portrayal of the effects of unlimited ambition. It is ambition, and nothing else, that leads Macbeth to commit the initial murder of Duncan, as he tells the audience immediately before committing the deed:
I have no spur
To prick the sides of my intent, but only
Vaulting ambition, which o'erleaps itself
And falls on the other...
Macbeth is not an unfeeling monster, though we hear from descriptions of the battle that he is capable of extreme violence. It is ambition that drives him to commit a deed which would have otherwise been unthinkable. Left unchecked, ambition sets in motion a string of events that leave several others dead, and ultimately destroys Macbeth. So Macbeth is in part a meditation on the effects of allowing ambition to overwhelm one's own moral code. By portraying him in a loving relationship with his wife, and as an honorable soldier and lord, Shakespeare invites the audience to consider whether Macbeth is that much different than them.