1) If no one sees him do it. Macbeth's desire to become king is driven by his ambition, but he is also very aware of the consequences of his actions. He knows that if he is caught murdering the king, he will be executed. Therefore, he takes great care to ensure that no one sees him commit the crime.
2) If he is not caught. Macbeth is also aware of the fact that even if he is not seen committing the crime, he may still be caught if someone else discovers the body. Therefore, he takes steps to conceal the body and make it look like the king died of natural causes.
3) If he can get away with it. Macbeth is confident that he can get away with murdering the king because he is a powerful nobleman with a lot of influence. He believes that he can use his power and influence to protect himself from the consequences of his actions.