For all the complaints about JJ recycling ANH, I think he does some really new stuff in this movie, and his depiction of Ren is one of those things. He’s a flawed villain, instead of a perfect villain. He clearly is strong in the Force but he can’t control it. His “temper tantrums” made perfect sense to me.
They made sense to me also. I really liked that scene where you expect him to force choke or otherwise kill that officer, but instead he throws a tantrum. That scene helped show that he isn’t just a clone of Darth Vader.
What made him look pathetic was the fact that Rey and even Finn were able to hold their own against him. Considering he is supposed to have killed a number of trained Jedi, and he has had actual training whereas Rey hasn’t, that cheapened his character for me.
We don’t yet know if he killed any Jedi who had a lot of training. He could have been one of Luke’s eldest students. He also likely had help possibly from the Knights of Ren or Snoke, neither of whom we know much about at this point.