I'm going to assume we're not on the same page, or you're not on the same page as OP. It's pretty obvious that what the OP defines as dark & twisted is something that tends to gravitate towards change for the bad. There's really no need to define specifically. If you follow the recent lore, you'd realize that half of the new cast have some sort of conflict or inner turmoil and it's always been that way for newcomers to the point where it feels bland.
Being realistic doesn't necessarily translate to having story elements that completely changes the character's personality or outlook, nor does it mean you have to throw some life changing events that scars the character completely. Quite frankly, the only thing the lore team has going on at the moment is ridiculously long chapters of lore without a specific direction. If we look back at the old style of lore, right when summoners were still an integral part of the game, one shortly written paragraph is enough to give players a glimpse of each champions' background.
Ironically, Riot has been trying to push for the imaginative, fantasy element among players, meaning its entirely up to you to interpret whether a champion is related to another, or that a champion is secretly from some other universe. Yet, right now, we've got the narrative team trying to make champions seem much more realistic.
Whether League's lore is getting much more realistic is not my concern, but the OP has a valid point in that every subsequent introduction to the newcomers have carried a grim, dark past. The tone in which most stories were written carry that same dark, solemn tone. Once in awhile you want characters like Braum or Lux whose story is on the lighter side, Braum is really just your jovial mid age guy who became the protector of his village, and Lux is your average girl with the brains of a prodigy who happens to wield light magic.
Garen is your standard hero who met his match on the battlefield, and is still heralded as Demacia's royal guard. Whether you think Garen is the true evil or that Darius is just another Noxian fighting to defend his country isn't the focal point. However you interpret that character is entirely up to you. If you wanna dig up some silly conspiracy that Garen is plotting to take over Demacia and rage war against the rest of Valoran for world conquest is perfectly fine too. The purpose of the OP is to point out that each subsequent lore carries the same tone, and it is frankly getting very very stagnant pretty soon. In fact, none of the lore since S3 were well written imo. The flow in the use of words isn't there and it almost feels like I'm just reading a story without enjoying it. And the narrative team has equated long written stories to a quality one, which isn't quite true.
A brief, succinctly written lore in a paragraph can give you just as much insights to a character than a couple pages of stories would. And the current stories are much better off as an expansion of their lore, replacing League Judgments as an insight towards a player's champion.
So yes, not all champions have to go through the same, dark, almost monotonous narration to get to where they are. If Riot one day decides to change up Lulu's backstory by somehow changing her in to this regular teenage girl who was bullied for being outcast and one day she discovers the fae magic and got turned in to a yordle sorceress, that would be so weird and unfitting. Some characters just don't have to go through that phase of narration to be realistic. You could start off Ivern as a non-bloodthirsty individual and you'd still have a good way of introducing him to the player base.