Yeah, i can understand people's frustration with the direction they took Luke's character... but if Luke saw into the "future" that Ben Kylo was down with murdering billions(i assume) of people on three planets with some ridiculous planet-sized weapon a la the Deathstar AND saw him kill his own fricking dad... aka Luke's old pal Han, then he might've turned into the Kenobi role(seeing no redemption in Anakin) and feeling the need to end him, although IT WAS a fleeting moment. You might say Luke drove him to those actions but c'mon... they seem to establish that Snoke already pretty much turned him so it was probably inevitable anyway.
You saw how it affected Obi-Wan when Alderaan was turned into an asteroid field.... dude nearly had a heart attack. Well imagine Luke thinking he could prevent Alderaan from happening on a larger scale and saving Han and all the other terrible things Emo Ren did in the movies and potentially off screen. It's plain to see with this and the whole hermit-act, they turned Luke into a Kenobi/Yoda hybrid which isn't far-fetched because heck, they were the ones that got the man to where he was, or at least a huge influence on him.
At the end of the day, you guys with the ultra hate-mongering are still stuck in this cycle of holding anything new up to the OT.... I stopped that after the PT and and have been able to garner some entertainment from the ST. I'm not saying you have to set your expectations low, just don't expect to have that same captivation you felt from the OT and you'll be fine. For me, the movie was not bad... I could've done without the whole Casino world sequence, but outside of that I can't say i have this compelling urge to have KK fired and for Rian Johnson to never work in this town again(thoroughly enjoyed his helmed episodes of Breaking Bad).
If George Lucas could see this thread, he might feel a bit vindicated though after not pursuing his own sequel trilogy after the backlash from the PT.