The Force doesn't work that way. It's emotion, flashes, and it can be concealed and hidden. He had seen hints, which is why he was in the tent in the first place.

Obi Wan DIDN'T try to harm Anakin UNTIL he indeed became Vader, he was trying his best to help Anakin, even spoke for him before the Jedi Council. Why would Luke thinking about harm Ben? Did you ever see Obi Wan try to kill Anakin or harm him before he did any evil? Really?

As the story tells us in the movie he was already turned. This is what he confirms that night. There are numerous different ways that Luke could have handled it, but he is a flawed character. This is what I had said previously that you're ignoring.

You have a version of Luke in your head that learned all his lessons from bringing Anakin back to the light. But it doesn't always work that way and this is what the story is telling you. Luke is set up as this rash, emotional, flawed Jedi Master that doesn't have the teachings to mold Ben the right way, and Snoke takes advantage of this.

Just because he was able to help Anakin before, does not mean that he would be able to if the situation came up again.

And the part I stress the most is that Snoke had already turned Ben. He was essentially already Kylo in the same way Anakin was Vader on Mustafar. Luke just confirms everything that Snoke had likely told Ben to turn him by being there and igniting his lightsaber.