Macbeth is ultimately responsible for his own actions in the play. A few people have a role in his decisions, but when it all comes down to it, he makes the last decision for himself. The witches first plant the seed of him becoming King. This happens when they first meet each other when Macbeth is on his way to Forres to meet King Duncan. After that seed is planted by the witches and Macbeth starts thinking about it Lady Macbeth plays an important role in him going as far as killing the king. She questions his manhood in an attempt to make him change his mind. Near the end of the play especially it starts to show Macbeth starting to corrupt his power as king. He also shows he only wants to remain king. When it comes down to who is the most responsible for the killings, the weight rests on Macbeth's shoulders.
The witches were partially responsible for Macbeth's actions. They were the ones who started the whole idea of him making his way to the top. It started when they said to him, "All hail, Macbeth! Hail to thee, Thane of Cawdor!" and, "All hail, Macbeth! Thou shalt be King hereafter." After Macbeth becomes Thane of Cawdor, because the previous Thane was being charged with treasons capital, he starts to realize that the witches could be right. Because of the witches impact on him, he kills everyone that comes close to threatening his place as king. When he goes back to the witches for a second time, they show him three apparitions. The apparitions all had their own things to say about things that would soon happen in Macbeth's life. Macbeth leaves the witches the second time feeling invincible and more untouchable than ever.