Emperor Palpatine was the head of the Empire, he created it from the Republic and without him at it's head, it crumbled to dust. Seeing as how easily the Empire fell once Palpatine was dead, why didn't anyone try to kill Palpatine sooner? We know of a few attempts, Headmaster Gentis's coup attempt comes to mind as the most successful, as he actually managed to mortally wound the Emperor. Palpatine would have died, had Vader not come to save him. Another, less effective attempt was Trachta's ill-fated coup attempt, which failed miserably. You would think that Trachta, having witnesses Gentis's attempt personally, would have had a much better plan for killing Palpatine. Who could have killed Palpatine and thus brought about the end of the Empire and how could this have been accomplished? Note: Yes, I know Vader managed it at the end, at the cost of his own life, but I'm thinking more in terms of other beings, like Rebel assassins, Jedi survivors, Imperial traitors, etc.