Luke almost killed his own father and fought out of anger all the fuckin' time. He force chokes a guy to death in rotj. He was always a grey Jedi, how is it so shocking to believe he'd have a fleeting moment of fear towards the darkness rising in Ben. Not only was it super powerful and scary, but also had the potential to destroy everything. And then it did! All because he ignited his lightsaber over his sleeping nephew. For just a second, he pondered doing something awful to prevent more and more even more awful things from happening, trading his nephews life for the safety of the galaxy. And then it all goes to shit because of that one tiny thought that popped into his mind because Luke wants to keep everybody safe, his friends, his family, his students, his temple, whatever.
He didn't ignite his saber just because of the darkness in Ben, he ignited it because he had so much to lose. His actions caused by his desire to protect everybody got everybody killed. How did the Jedi ever let Luke down? Yoda and Obi-Wan gave him everything he ever wanted, adventure, an epic life beyond being a farm boy. A Jedi seeks not these things, but we all know Luke did. The Jedi failed him just because they lied about Vader? Big whoop.