Please note that eNotes rules do not allow for the answering of multiple questions. Please repost your remaining questions separately.
In Macbeth, this soliloquy in Act III, scene i develops Macbeth's character as his "vaulting ambition" (I.vii.27) takes over from all reason. From deciding against even killing Duncan "We will proceed no further in this business"(30), Macbeth is now intent on removing anyone who may prevent him from becoming king. Macbeth is lamenting the possibility that Banquo's sons may take the throne as the witches "hail'd him father to a line of kings."(III.i 59) This would mean that "For them the gracious Duncan have I murder'd."(65)
Macbeth's words also develop the plot as Macbeth, who initially depended on Lady Macbeth's harsh talking and her casting aspersions on his manhood when she suggested,"When you durst do it, then you were a man," (I.vii.49) becomes more determined to attain and retain the crown. He is now king and knows that Banquo thinks "thou play'dst most foully for't"(III.i.3) and, even without Lady Macbeth's urging, believes he is invincible. His fear drives him and he intends to have Banquo murdered. Importantly, the audience notes Lady Macbeth's exclusion from his decision to kill Banquo. This will become relevant as her guilt will lead to her ultimate madness and death.