Whoever said you can't judge a book by its cover never laid eyes on the Emperor. From his black, hooded robe to his droopy skin and yellow eyes, everything about Emperor Palpatine's appearance oozes malevolence. "This character [is] solidly evil," explains Ian McDiarmid, the actor who plays the Emperor; "it wasn't as if he had a difficult childhood. He just seemed to have come from whatever ghastly womb he was ejected from, born and bred evil."
It's Good to Have Goals… Unless They're Evil
The Emperor has three twisted goals he wants to accomplish in Return of the Jedi. They're all interconnected, and they're all motivated by his wrinkly lust for power.
First, the Emperor wants to turn Luke to the Dark Side just like Darth Vader:
THE EMPEROR: Come, boy, see for yourself. From here, you will witness the final destruction of the Alliance and the end of your insignificant rebellion.
[Luke eyes his lightsaber]
THE EMPEROR: You want this, don't you? The hate is swelling in you now. Take your Jedi weapon. Use it. I am unarmed. Strike me down with it. Give in to your anger. With each passing moment you make yourself more my servant.
THE EMPEROR: It is unavoidable. It is your destiny. You, like your father, are now mine.
The Emperor's confident that the capability for corruption is part of the Skywalker DNA.
Speaking of Luke's father, let's take a second to look at Darth Vader's role in the Emperor's nefarious plans. The Emperor may call Vader his friend, but, really, he's just using him. Vader is the Gretchen Wieners to the Emperor's Regina George—part bait and part guard dog. Knowing that the Skywalkers can sense each other, the Emperor just hangs out by Darth Vader and waits for Luke to visit his dad. Then, when Luke finally shows, the Emperor takes a seat and sends Vader in to do his dirty work. While Vader and Luke duke it out with their lightsabers, the Emperor chills on the sidelines, like the ugliest cheerleader in history.
Why is The Emperor so hell-bent on turning Luke to the Dark Side? Luke's just one guy, right? The Emperor has two reasons: first, the Force is super-strong in Luke. If the Emperor can get Luke to break bad, he'll be a huge asset to the Empire. Second, bringing Luke onboard Team Dark Side would help the Emperor accomplish his next goal: crushing the Rebel Alliance's morale.
In Return of the Jedi, there's a war being waged between good and evil. Luke's the #1 draft pick for both sides. If the Emperor turns Luke, it won't just bolster his squad, it'll deliver a painful blow to the Rebels. Luke's fellow members of the Rebel Alliance aren't only his teammates; they're his buddies. Seeing Luke turn into Darth Vader, Jr. would really mess with the collective head of the Rebel Alliance.
Which brings us to The Emperor's final goal: He wants to wipe out the Rebel Alliance, period. It's textbook villainy. The Emperor wants to rule the entire galaxy, and he won't let anybody stand in his way.
President of His Own Fan Club
Ultimately, the Emperor's arrogance is his downfall. Exploding to death in a reactor shaft played a pretty big part in his demise, too… but it's his overconfidence that got him there.
Like many a classic movie villain before him, the Emperor has a big mouth. He has despicable plans, and he just can't help but explain them to everybody 24/7. When we first meet him, he's reminding Darth Vader that Vader's the first step in the Emperor's scheme to corrupt Luke:
"In time, he will seek you out and when he does, you must bring him before me [...] Only together can we turn him to the Dark Side of the Force."
The Emperor has no interest in Darth Vader himself; he's just another cog in the Emperor's cocky machine. That's precisely why Vader has no problem turning on the Emperor in the end. Would it have killed the Emperor to welcome Vader's input, or compliment his shiny mask once in a while?
Similarly, the Emperor has no interest in what the Rebel Alliance is actually doing. He's only interested in his own plan, and, in his arrogance, he's never considered that it might not be a very good one.
For example, you'd think the Death Star's defensive shield being blown to bits would send the Emperor back to the drawing board, pronto, to reconsider his strategy or launch Plan B. Instead, what does the Emperor do? He ignores the Rebels' monstrous blow to his strategy and, instead, invests his energy in telling Luke how much smarter he is. He sneers at Luke while the Empire is in utter chaos:
"Young fool, only now, at the end, do you understand. Your feeble skills are no match for the power of the Dark Side. Now, you will pay the price for your lack of vision."
Ironically, it's the Emperor's lack of vision—or at least his inability to get over himself—that ruins the Empire. His giant ego prevents him from accepting others' help and ideas; it leads him to craft a flawed plan for the Rebels' annihilation, and it inspires Darth Vader to turn against him.
In short, the Emperor may be the baddest dude in the whole entire galaxy, but he sure ain't the smartest.