The reason his ambition led to him killing Duncan was because of Lady Macbeth. she was like the fuel for his hatred and so some people may have felt sorry for him as it was not COMPLETELY his fault and he did regret it later on in the story. however, some people may believe that if he had a stronger character, a character that was perhaps more 'Masculine', he would have been able to stop himself from being influenced by his evil wife. personally, when he killed Duncan, I had sympathy not for what he did, but how he didn't have enough willpower to stop himself.
