It can vary. I'm talking about situations when someone's motivation of rising to power is protection, so they can help other people when it's need. It also can be someone who had a chance but didn't want to be in position of power and changed their mind because it's only way to protect someone/prevent something bad. Revenge or money or power itself shouldn't be a motivation.
Examples:
Moon Lovers - Prince Wang So and his coup because of Had Soo
Doctors - SML didn't want to be in director board but change his mind to prevent FL's firing
Under the Queen's Umbrella - MLbecome next crown prince to protect his mother and brother's family