They would not have us believe he'd murder his own nephew. Because, well, he didn't murder his own nephew. He didn't even try. The movie suggested he had a passing dark thought that lasted literally only a second and he came to his senses. Problem was that one second of a passing dark thought had major consequences.
There's at least the argument to be made about Luke's isolationist behavior. I get that. I subscribe to some of it. But this never had a leg to stand on and still doesn't. It actually fits his character. The temptation and brief touching of darkness only to relent. Its happened before. And to squash a common rebuttal immediately, "He's older now and should have learned his lesson" doesn't work. Because that is not how the dark side of the Force works. A Jedi doesn't just "learn his lesson" from one temptation and that's it. Even the most dedicated Knight has to always be vigilant to not let it creep in, all the time. There is no point of immunity. That's just one of the realities in this mythos of utilizing the Force.