I have a very simple theory about the Sentry.
Osborn and the Void used to hang out.
Crimelording it up.
Osborn knows how to kill the Void.
(It's to do with the Hulk. If he Sentry makes him weaker, then the void will make him stronger, but no ones told that to Bob yet.)
Perhaps Osborns insanity protected him from the Sentry whiping his adventures from realities recall?