The guy has more advanced Force powers than any Sith or Jedi seen in the series yet (he can hold a laser bolt in mid air for many minutes), that and he still has the emotional range of a frustrated teen in his thirties makes for a potentially prickly and unpredictable villain.
That’s what I like about Kilgrave in Jessica Jones he has such power he could do anything and instead he just forces people to make him dinner and convince children it’s okay to piss in the closet.
Vader had a vision of a galaxy without the need for conflict. He would destroy all resistance, including his boss. He would rule the galaxy with his son and all the killing and pain would have been worth something.
Kylo is like someone trying not think about porn and failing in repeatedly embarrassing ways.
TROOPER ONE: I’ve got a message for Kylo Ren
TROOPER TWO: Don’t go in there, he has been trying not to think about porn for five hours, the place is going to need detergent and a squad of washbots