How is Macbeth’s ambition bad in the play?

Macbeth and his ambition is definitely a source of discussion from this Shakespeare play. I would not go so far as to say that all of Macbeth's ambition is bad, evil, etc. I think it would actually be a negative trait if Macbeth (or anyone) had zero ambition. What is bad about Macbeth's ambition is that it is unrestrained ambition.
Macbeth begins the play as a well-known, well-liked, and wealthy thane. His battlefield exploits causes him to be promoted to Thane of Cawdor. He is happy about this, but the promotion does remind him of what the witches predicted. They said that he would one day be king, and now Macbeth is one step closer to the throne. Like any normal human being, he considers what being king would be like. He desires the throne, and most other men in his position would also desire it. What becomes burdensome to Macbeth is that he envisions a pathway to get him to the throne in a shorter amount of time if he takes certain actions. Again, there is really nothing wrong with that information. The problem is that his shortcut to the throne involves committing regicide. Assassination and murder are morally corrupt actions, yet Macbeth's ambition and desire for the throne cause him to seriously consider going through with the plan. His ambition is bad because it is stronger than his moral compass.
To be completely fair to Macbeth, he does decide to not kill Duncan. Macbeth knows it is wrong, and he knows that Duncan is a good king as well. Macbeth tells Lady Macbeth that he will have no more to do with killing Duncan, and Lady Macbeth goes into one of the greatest speeches in all of Shakespeare.

What beast was't, then,That made you break this enterprise to me?When you durst do it, then you were a man;And, to be more than what you were, you wouldBe so much more the man. Nor time nor placeDid then adhere, and yet you would make both:They have made themselves, and that their fitness nowDoes unmake you. I have given suck, and knowHow tender 'tis to love the babe that milks me:I would, while it was smiling in my face,Have pluck'd my nipple from his boneless gums,And dash'd the brains out, had I so sworn as youHave done to this.

She berates and insults Macbeth, and eventually he changes his mind and goes through with the murder. I'm not 100% convinced that Macbeth kills Duncan out of sheer ambition or fear of his wife or some other reason, but I am convinced that his unrestrained ambition is what leads him to continue having people killed after he obtains the throne.