Macbeth thinks he deserves to be king. Yet he is also very wishy-washy. He is not as ambitious as his wife, for example. There is definitely a scene or two with him questioning what he has as soon as he has it. He worries enough to kill both Banquo and Fleance, and then Macduff's family.
Macbeth is at first unsure of himself and if he could actually go through with killing the king. However, with his wife's help, his ego grows and his conscience dies. He sees himself as the ultimate leader once he reaches that point and will do whatever he has to to maintain his position as king. He sees no other way and is willing (and doesn't care) to kill anyone in his path, even the Macduff children/family.