It’s because the Emperor is so arrogant and mad with power that he doesn’t feel the need for subtlety. He’s a sadist who gets off on his power and control and evilness. Luke flat out says “Your overconfidence is your weakness” and that’s 100% true. I don’t see what’s so hard to understand.
Exactly. He’s telling Luke that he’s going to fall to the Dark Side because, as far as the Emperor is concerned, “it is unavoidable”. Nothing can stop it, he has foreseen Luke defeating Vader and taking his father’s place at his side so he’s letting Luke know that it doesn’t make any difference whether he resists, because it is his destiny.
We already know the Emperor is a master manipulator and that he sees the future. He deliberately let the Rebellion know where the second Death Star was and that he’d be there. He tells Vader that Luke will surrender himself to him to Vader’s disbelief, and yet, lo’ and behold, Luke does just that. He knows that the shuttle Tyderium was a Rebel ruse. He foresees Luke falling and as far as we’re concerned, we see Luke start down that path when he loses control and attacks the Emperor, which Palpatine sees as Luke fulfilling his destiny to join the Dark Side and replace Vader. If anything, that makes him a better villain: he’s spot on about everything so far, and then even Luke seems to fall into his trap.
He’s scary for the same reason slow zombies are scary: the inevitability.
Like Density said, the only reason his plan failed was because of his overconfidence and underestimation of the Rebel resolve against the Empire, Luke’s resolve against the Dark Side, and finally (as well as fatally) Vader’s resolve. When Luke defeats Vader, realizes where he is headed if he continues, and throws down his weapon to show that he is truly a Jedi, the Emperor realizes that he needs to up the ante, and so he tortures Luke in an attempt to break him, to turn him to the Dark Side by Force (hur hur). Again, he was overconfident in his ability to do so, in Vader’s allegiance to the Sith and his master, and he underestimated the good inside of Anakin and the love a father has for his son.
That’s how I see it at least.
Hell, he might have even built the second Death Star not to actually use on the scale of the original Death Star, but as a massive trap. Nothing better to give your enemy a false sense of hope than to pit them against a familiar scenario they won before, but change the rules. He was stacking the deck against the Rebellion the whole time, but failed to account for their spirit.