But you're overlooking that the Force enables sensing imminent attacks and danger. That's literally the very first Force ability Luke was ever trained in by Obi-Wan. It also gives telepathic or empathic abilities, like sensing people's emotions. Luke and Ben had been training together long enough that they must have known each other's minds pretty well, and Luke's intent to kill and his inner turmoil at doing so would've been radiating off him intensely.
