The Emperor’s insistence on being there to witness the Rebellion’s defeat is a form of unintentional self-sabotage.
He spent his whole life on this conquest for power, and when he finally gets it… there’s no thrill. He spends all day sitting in a chair. How boring is that? He needs to put himself in positions where he can get a new high off of showing off his power.
That’s why he needs to be there for the Battle of Endor. He needs that high, because otherwise he’s just feels empty. And it’s ultimately his undoing.
Many say Vader got the short end of the stick in having to do the Emperor’s dirty work, but what I find interesting, especially in Canon, is you can tell that Vader does get a thrill out of his missions. He likes to show off his power, like the Emperor.
The Emperor and Vader are essentially in a toxic codependent father-son relationship: they can’t live without each other unless they have a replacement (for both, this is Luke). Otherwise Vader doesn’t have a mission, and the Emperor doesn’t have a throne to sit on. You can’t have politicians without enforcers, and you can’t be the Sith Master if you’re not the Master of any Sith. Vader may be a slave of the Emperor, but in a twisted way, the Emperor is also a slave of Vader. What’s he gonna do without him?