The newest trailer for Star Wars: The Last Jedi has a lot of people talking and trying to figure out what it all means. Some of it, however, seems to be more clear than the rest. One shot shows Luke Skywalker and R2-D2 standing before a burning structure, and another, a moment before that, seems to show Luke climbing out of the rubble of the same building. Based on comparing this to other things we've seen it appears that Kylo Ren came very close to killing Luke Skywalker. This may explain why Luke felt he needed to hide, he was dead if he didn't.
The image above from the Star Wars: The Last Jedi trailer appears to show Luke Skywalker kneeling next to R2-D2 as they watch a building burn. Based on the positioning, it seems very likely that this image is a second angle on an image we've already seen. The closeup of Luke's hand with the droid from Rey's vision that we saw in Star Wars: The Force Awakens. They are clearly sitting before a significant light source in that image too, so it appears that light source, as IGN points out, is the burning Jedi Academy, as we know that Luke was training several new Jedi before Ben Solo turned on him.
However, the other important image from the trailer, that would appear to come from the same point in time, shows that, before standing in front of the burning building, Luke Skywalker was actually in the burning building.
If the Knights of Ren set the building on fire with Luke Skywalker inside it the intention was certainly to kill him. Who's to say they didn't think they succeeded? At the very least, this would explain how Luke was able to escape and go into hiding undetected. The fact the nobody knows where Luke is could be in part because everybody thought he'd died in that fire. It's clear that by the time of Star Wars: The Force Awakens it's a known fact that Skywalker is alive, but if they tried to kill him once, they certainly would have continued to try unless they thought they'd succeeded.
This could also be why the trailer implies that Kylo Ren will attempt to kill General Leia in Star Wars: The Last Jedi. Ren is trying to sever the ties to his old life, and she's the biggest one still alive. Perhaps that's what Supreme Leader Snoke meant at the end of The Force Awakens that it was time for Ren to complete his training. Perhaps Snoke wanted Ren to kill those that raised his former self. He tried to kill his teacher, successfully killed father, now it's his mother's turn, probably prior to going after the teacher once more.