An aside is a dramatic convention in which characters voice their thoughts, but other characters on stage cannot hear them. Why does Macbeth begin to talk about murder in his aside in lines 129-141? Whose murder?

Because he has received news that he has become the Thane of Cawdor, he begins to consider that the witches knew what they were predicting and that they were right.  However, the possibility of being king is a good thing, on the one hand, but he is thinking "If good, why do I yield to that suggestion/Whose horrid image doth unfix my hair/And make my seated heart knock at my ribs,/Against the use of nature?"  In other words, he is thinking about murdering the king and that is a terrible thought.