Duncan and Banquo, however, he is murdered onstage. To haveMacbeth's assassins stab a child in front of the audience's eyes brings home, as words cannot, the horror of what Macbeth has become. There is no reason for Macbeth to have this child killed: it will be years before he can pose any realistic threat to the throne. Having him murdered shows that Macbeth has become a despot, whose actions make him utterly unfit to rule Scotland. Macbeth's thought before he kills Duncan, that one murder will lead to another until he is drowning in blood, has come true.

Further, the boy's awareness, shown through his dialogue with his mother, that the world is a corrupt and evil place shows the ill effects of Macbeth's reign. His mother says that liars and traitors must be hanged. The boy states to his mother that the liars and traitors of the world outnumber the honest enough to turn the tables:

there are liars and swearers enough to beat the honest men and hang up them.

Macbeth's initial murder of Duncan has had effects that reverberate across Scotland, unleashing evil to the point that even children are aware of it.

In act 4, scene 2, Macbeth sends several murderers to slaughter Lady Macduff and her family while Macduff is in England petitioning Malcolm to assemble an army to reclaim Scotland. This scene is significant for several reasons. By slaughtering an innocent woman and her children, Macbeth's depravity is complete, and the audience recognizes that he has transformed into a maniacal, ruthless tyrant. Macbeth has no concern for human life and is determined to punish his enemies and anyone associated with them. In addition to emphasizing Macbeth's bloodlust, this scene also establishes the rivalry between Macduff and Macbeth and pits them against each other in the final battle. Macduff is additionally motivated to kill Macbeth and must avenge the deaths of his beloved family.

Unlike Macbeth's previous murders, the slaughter of Lady Macduff and her family seems senseless and irrational. Macbeth committed regicide to usurp power and murdered Banquo in an attempt to cement his legacy. However, Lady Macduff and her children are murdered for no reason other than to satisfy Macbeth's bloodlust. He has nothing to gain by murdering Macduff's family and only motivates his enemies to seek revenge. Also, Macbeth's decision to kill Macduff's family is impetuous and rash. Macbeth does not contemplate his actions or think about the consequences attached to the murders. He simply gives the order to have Macduff's family slaughtered and shows no remorse for his actions.

The scene serves several purposes. As your question suggests, it provides a way for the audience to see how Macbeth's evil has grown from when he first kills Duncan to this moment now, when he orders the murder of a defenseless woman and her children in order to terrorize Macduff, his enemy.  His other crimes served his "vaulting ambition," but this one does not serve such a purpose for Lady Macduff and her children are not his adversaries.  The scene also serves the dramatic purpose of providing a domestic scene, a woman with her family, that provides a stark contrast to Lady Macbeth, who lacks all qualities of a mother, which we see when she says to the evil spirits "Come to my woman's breasts / And take my mild for gall" (1.5.54-55).