The witches (three weird sisters) gave Macbeth and Banquo the premonition that he (Macbeth) would become Thane of Cawdor. This comes true, so Macbeth informs his wife in a letter of this promotion. He also reveals to her the other premonition and this is that he will be king. So, he and Lady Macbeth conspire to kill Duncan in order to speed up that process. With Macbeth as king, they will then be in the most powerful position in the country. From that vantage point, they feel they will be able to gain anything else that they might want. In other words, they gain a belief and/or power that they will be able to gain even more.
Once Macbeth is king, he feels that he should be able to get away with murdering Banquo and Fleance. This prevents their descendants from becoming king, making it more likely that Macbeth's sons will be kings after him. Once Macbeth and Lady Macbeth achieve a certain amount of power and authority, they need to have more.
So, they gain the crown and control of the country. They gain tremendous power from the royal position. They gain fear and respect from their thanes, lords, and subjects. They gain fame and fortune which come with being the most important public figures. And they gain such powerful positions as to afford them the opportunity to gain even more power over others.