Zo'hvaal (The Jailer) was likely turned evil because they ended up pretty much scrapping the whole shadowlands expansion, So they needed a villain and he was the easiest bet.. If we look at the early concept art of the jailer, who we know as the Runecarver (primus) would actually have been the original jailer. So theres definately a possibility that the guy we now have as the jailer, wasnt originally evil at all. and yes, its a shame that the story was told the way that it was :/ I somewhat think blizzard really just wasnt prepared to explore the afterlife part of the warcraft universe, and took on too much..
The chronicles was for the most part good indeed, But even lately the game has strayed away from the chronicles, and as some lore nerds (Like Pyromancer before he started hating on wow, and accolonn) has pointed out, the chronicles are all written from one point of view (i think this was mentioned by danuser in an interview at some point?) and in this case, that point of view would be the titans.. and with that in mind it means that we are only seeing what the titans see as being "The True Story" and what they decide that we are allowed to know, with likely many more forces etc out there, and likely alot of things they say are "evil" not being as evil as they are portrayed to be.