Macbeth does not start off the play as a ruthless, dispassionate murderer. He begins as a lauded warrior, defeating enemy upon enemy and earning new titles in the process. He is loyal to his king and loving to his wife.
So what happens? Well, he meets the witches, and they plant the seed of the king's crown in his mind. Yet, he still mulls the idea over, initially deciding against murdering Duncan and even presenting reasons to himself why he should not even consider such a horrible idea.
Unfortunately, with his wife's relentless badgering, he agrees to the murder. From that point on, he must continue killing, lying and plotting to keep his position. He comes to rely so much on the witches next set of prophecies that he lets his relationship with his wife and his logical and rational thought processes to slip, resulting in his wife's and his own tragic deaths.