He doesn’t exactly try to kill Ben. Kylo’s version of the flashback with Luke’s crazy face and swing at Ben is implied to be untrue, like Luke’s original telling of the story - both are exaggerating to make Rey more sympathetic to them - but Luke’s second telling seems to be the truth. He pulls his saber out in a moment of shameful fear and weakness (he is, after all, a Jedi, and pulling their saber out is usually a reasonable thing to do instinctively when they sense a threat) and then, while staring at it in shock, is forced to use it to block Ben’s incoming strike. It shows progression from ROTJ, in which Vader’s threat against Leia prompts Luke to angrily fight Vader across the room and chop his hand off. He’s gotten better at controlling that instinct to lash out, and catches himself much sooner, but the tragedy is that it still wasn’t good enough, and he’s forced to live with the shame of knowing that (until he learns to forgive himself in the movie).