The Dark Side clouds everything. While the Emperor was extremely tactful, cunning, and patient, he was also so consumed by the Dark Side, especially during his Force Lightning attack against Luke that he could not sense Vader's betrayal; while he reasoned, as others pointed out, Plan A had failed and Plan B would be seen to fruition.
Palpatine told Vader, "He(Luke) could destroy us." He essentially implies that if Luke becomes a Jedi, he would be more powerful than both of them, maybe not combined, though I don't believe it's revealed what Palpatine's vision was, he certainly senses Luke's power(I had reasoned Luke was shielding himself from the Force(Apparently "shutting himself off"(TLJ)) when they were headed to Endor and Vader sensed him and the Emperor did not.
The Sith seek power above all. The Emperor was no different. Anakin could have killed Palpatine, he even asked Anakin if he was going to kill him, and Anakin certainly would like to, but doesn't, instead he turns and joins Palpatine. Perhaps the Emperor thought the same would happen with Luke.
Probably the biggest issue right now is that Luke, who believed there was still good in his Dad and even faced the Emperor and near death(watch ROTJ again the TLJ), figured in a bad choice moment(i guess we all human, but...seriously?) went to kill his nephew. Now, of course Luke wouldn't go tell Leia and see if she thinks that's a good idea, right? Don't get me wromg, I love the story of Luke Skywalker...except how it ended. The conclusion to his story is in the hands of one person and sadly it didn't turn out the way it should have. Among many other things, besides training Rey in the Force...the Force, not the Jedi, the Force, life creates it, makes it grow, c'mon that's not that hard, he should have gone with Rey, Chewie, and R2. I get why he didn't, but why not, the writer wanted to leave his mark on the Star Wars universe? That's the only explanation I can come up with, because the Luke I knew would have followed in the footsteps of Obi-Wan from ANH and gone and fought, even though Obi-Wan represented the Jedi, and it's time for the Jedi to end, I'm the last Jedi, I came here to die, the Jedi end with me, legacy of the Jedi is failure.......wait for it.....I'm not the Last Jedi, she is.......ummm.....was that supposed to be a plot twist, or did he just put the Failure Legacy on Rey? I don't believe it...and perhaps that is why I fail. Adventure? Heh. Excitement? Heh. The Last Jedi craves not these things. Here's hoping we see the real Luke, well real Force Ghost Luke in IX. Oh, and I don't know much about JJ, but if folks think he redid ANH with TFA, wellllllll...... The Last Jedi < Empire Strikes Back (couldn't bring myself to put an equals sign) but serious, same movie, different characters, imagine Vader killing Snoke in Empire and then Luke going to Bespin amd saving his friends while Yoda Force Projects himself against Vader.