Violence in Macbeth is central to action. The play begins with a battle against rebel forces in which Macbeth distinguishes himself as a valiant and loyal warrior. Later, Macbeth's murder of King Duncan is condemned as an unnatural deed but the play also raises the question of whether or not there's any real difference between killing a man in combat and murdering for self gain. Violence in all forms is frequently associated with masculinity – the play is full of characters (Macbeth, Macduff, Young Siward, and so on) that must prove their "manhood" by killing. Even Lady Macbeth asks to be "unsexed" so that she may be "filled with direst cruelty." At the same time, the play also suggests that unchecked violence may lead to a kind of emotional numbness that renders one inhuman.
The reason that Macbeth's violence is inexcusable is because it doesn't play by the established rules. In Macbeth, organized violence is sport, and individual violence is uncivilized.
Throughout Macbeth violence and cruelty are associated with masculinity.