Either way, the funny thing is? There was a way you could have made her path of darkness be justified, how? Simple, this is her revenge against America's apathy towards the news of her hometown getting destroyed. I always imagined that Carlito knew gathering evidences and spreading the story to the media was pointless, he knew that since it wasnt in their backyard but in some South American "dump", Americans wouldnt give a shit.
It's that and resentment towards Frank and Chuck for not rescuing her in Case West. Remember, last time she saw them, they were being held at gunpoint and the flashdrive she gave them was smashed by giga-pajeet. You could argue that either Frank or Chuck should have distracted the guards and try to save Isabella, but at the end of the day these are just two dudes way in over their heads. Fighting zombies is one thing, fighting armed security guards on their own home turf is another. I guess she overestimated Frank, beating Brock hand to hand being her last memory of Frank must have build unrealistic expectations of him.
Don't forget that she was tortured, psychologically broken and later groomed by Marilyn. 11 years of isolation, since Dead Rising 2 takes place in 2010 and Dead Rising 3 takes place in 2021, she could have ended up being even worse.
I get that from the narrative standpoint it feels lazy, but it makes sense from the real world point of view. Ever since Santa Cabeza happened, she lost her family and she spend over a decade being groomed by Carlito to hate Americans. She later has to work with one as her own flesh and blood shoots her(probably not the first time he lashed out at her), Frank leaves her to be arrested and ends up not being able to prove America's involvement, she spends 4 years working in a lab against her will, and her one chance at freedom and to get the facts out is ruined because she expected too much out of Frank. 11 years is a good amount of time to build up hatred over someone, be it rational or not, and plan out what you want your legacy to be once you accepted your situation. At some point Isabella must have simply dealt with her new life as best she could and wanted to atone for her brother's sins, so she could finally start a new life and a new career. At that point, she didn't care about "exposing the truth", hell she didn't even cared how many people died, it's clear that Marilyn must have rubbed off on her by then.
I guess they had something planned for her, either a new villain arc or that planned Chuck+Frank DLC where she would show up as support or antagonist and air our her grievances. You can tell that her reaction to Chuck at the end of the game is rather cold. Either way, we will never know, so it's just a kinda awkward plot twist. I don't mind it, for the reasons I already described, but I can see how this could rub other people the wrong way. I just see it as a natural progression if we treat these characters as people, just like how Frank became fat and full of himself in OTR and how Chuck's personality eventually turned both Stacey and Kathey away from him.