first he was like "woah no way" but then lady macbeth was like"prove you are a man" and she started to cry so he said ok.
he (macbeth) feels guilt as he goes to kill duncan, he knows duncan is the king and he feels loyalty towards him.
she (lady macbeth) desires the kingship for him and wants him to murder duncan in order for macbeth to obtain it. not only that, but macbeth has a weak will, and lady macbeth, being determined and intelligent, manages to persuade the indecisive macbeth to commit regicide.
macbeth murdered his cousin duncan, whom was also the king, in order to become king himself. at first, he knew that it would not only be wrong to kill the king, but even worse to kill his own kin. but lady macbeth, macbeth's wife, wanted him to have this higher power and talked him into it.
macbeth felt guilty after killing duncan because he says something like no matter how much water he uses to wash the blood off of this hand it will still be there. he regrets what he has done. "wake duncan with thy knocking! i wish thou couldst".
macbeth killed king duncan for many reasons. one of the reasons macbeth kills duncan is because three witches tell him a prophecy that he would become the next thane of cawdor and eventually become king. after the witches' prophecy comes true about macbeth becoming the thane of cawdor, he is tempted by wicked thoughts to murder king duncan and fulfill his ambitions become the next king of scotland. the second reason is that macbeth is driven by lady macbeth to become the next king of scotland. once lady macbeth finds out the three witches prophecy in a letter about macbeth becoming the next king of scotland, she becomes lustful for the power and position to become the next queen. she urges him to kill duncan so that he could retain the title of kingship and she constantly encourages macbeth to kill duncan and tells him that it is not a really big deal.
Duncan is a guest in my house and I should be protecting him, not harming him myself.