The Skrulls have been testing and studying the Sentry for years, trying to understand how to best replace him. The plan they have is to leave him be, counting on his mental instability to take himself and those closest to him out of the picture. After all, the power of this man's mind created the Void, a creature with identical abilities to him that does the opposite of whatever the Sentry does, counterbalancing whatever good Sentry contributes. The Skrulls successfully make the Sentry snap, fleeing the planet, as seen in "Secret Invasion #2."
The Void, as usual, does the opposite.
