You forget Vader helped the Empire with the construction of the Death Star.
I think it's more than that. I think a certain amount of viewers probably had shitty childhoods and can identify with Kylo Ren. Even if they weren't that terrible they still feel resentment towards them.. I'm sure a lot of people here have great relationships with their parents but aren't there a few who didn't blame them for a lot wrong with their lives? Even just a little? Hell Carrie Fisher felt that way about her mother Debbie Reynolds and was distant from her for years. A lot of Loki fans hate Odin and Tony Stark fans really hate Howard. You'd be surprised how many Kylo Ren fans blame Han and Leia just as Han and Leia fans think Kylo was a spoiled brat who betrayed his loving parents. I have no doubt Leia and Han deeply loved their son but I also can easily believe Han would have trouble being a stay-at-home dad and that Leia would be too busy rebuilding the Republic leaving young Ben to fall prey to influence of Palpatine. The guilt they both feel in TFA didn't come from nowhere.
I just think you can't look at the movies in a real world way. The only way to really look at it is metaphorically. If you see Vader and Ren's redemption as asking the viewers to forgive mass murderers then it's terrible. If you look at it as a metaphor for parents(Vader, Han and Leia) sacrificing their lives to save their children, it's beautiful. Ben Solo's arc is basically Beautiful Boy with Timothy Chalamee and Steve Carell but with drugs instead of the Dark Side.
As to who should have been the main villain of the sequel trilogy, Kylo or Palpatine, it all depends on what you think is really ruining the world, the old in power who are clinging to an outdated status quo or the young who are under their influence and misguided.