Firstly:
So cool! Way to go! Beat their ass!
mc of original story smirks while doing exactly like in the story best route
Looking evil there! Entitled bitch! Kill her!
Secondly, in the story it wasn't revealed that Keith was dragged in neverending usury, just he betrayed Claude and made him vulnerable. He wasn't posed as innocent before Claude killing him and transforming into a dragon, sealing his fate. Otherwise Lilia as a character would have an option to stop it.
While choosing Claude route, there's also no information about why and how much Demon King actually liked the monsters, it's said the divide along with cold war was due their violent nature.
Him sealing monsters away from humanity seems beneficial, they're left alone and humans weren't attacked, it isn't explained it could have negative impact on the Demon King. Again, without knowing that, there's no way for fmc Lilia to prevent that negative impact.
Although, explanation could be the information was somewhere very deep in game, some tricky speech tree or note in not so obvious place where mc Lilia in game or Aileen as a player didn't have a chance to obtain.
There's also possibility it's all revealed later in the ending titles, nevertheless it was impossible or very hard for player to know beforehand or there was no alternative path set by game developers.
It's goddamn stupid to say how "useless the heroine was" if the heroine was
you playing the game wrong, freaking dumbass.