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).
Luke already conquered that darkness in Return of the Jedi. He saw his mistake, tossed his lightsaber and said that he’ll never turn to the dark side, knowing that it isn’t the way of the Jedi. Both scenes are the same: Luke is motivated to kill a family member because he fears that something bad is going to happen, but he stops himself from successfully killing said family member.
Luke was also less mature in age and the Force in Return of the Jedi, and he was in his 40s at the time he considered the cold-blooded murder of Ben Solo. Yoda even tells Luke in The Last Jedi that he has still been looking to the horizon, implying even more that Luke never actually matured as a person since The Empire Strikes Back.
He also would not have entered Ben’s room with a lightsaber at all, after growing as a Jedi for almost thirty years. Did he not learn this lesson when he decided to take his weapon with him in the Dagobah cave despite Yoda telling him not to do so? What did throwing away his lightsaber at the end of Return of the Jedi symbolize, other than that Luke has conquered the darkness? He had also learned about visions in The Empire Strikes Back. He should have moved on from that, and the sequel Disney trilogy should not be about Luke repeating the same mistakes.