IIRC "disease" was a mistranslation, she has an issue that makes her go crazy and it's not exactly explained why. It could be just extreme greediness or a mental disorder but I don't think Oda wants us to sympathize or to not blame her, she doesn't show any sign of regret after destroying the city or killing her son, after she goes back to normal. In any case, she uses her children and crew members like tools for her goals and that's the thing Luffy despises the most. She is also behind Zeff and the handcuffs, the Vinsmokes are just using it as Big Mom suggested. I'm kinda skeptical about her playing nice, like Brulee said, she was toying with them in the seducing woods, just giving false hopes so to crush their hope later, she is really clever and sadistic. There is also the issue of Fishman Island, Luffy said he would "free" the island from her, so I think he will be the protector sooner or later. That said, she could become an ally but that would make Luffy revise his morals and his "pirate way" due to these desperate times.
"Disease" was not a mistranslation, though it could be more along the lines of "disorder." The reason why Big Mom acted like nothing had happened is because she retains no memory of her actions when she becomes enraged.
That's why sweets are so important to her and her subordinates, cause they keep Big Mom sane. In exchange they get protection as well as an utopian society with no discrimination and no oppressive rulership like the World Government provides. Yes she has to teach the islands that don't provide those ingredients a lesson, but it's a cost to keep the land of Totland in shape as well as keep Big Mom's protection actually valuable. Just like in a real world a bank can foreclose your house for not paying up on the loan, Big Mom has her own form of upkeep that's sadly more violent.
In comparison the World Government literally slaughters and enslave nations for no real reason other than to save their reputation and cover up the truth they're afraid of leaking out.
Don't get me wrong it's far from ideal and perfect, and I'm sure her subordinates and citizens are fully aware of that, but it's not like any of them are forced to work with Big Mom. They're under Big Mom because they CHOSE to be with Big Mom, and that includes Fishman Island. Even in the Fishman Island arc when the fishmen were scared shitless after they learned that Luffy ate all of Big Mom's candies already knew there could be dire consequences for not keeping their upkeep. Basically, if they knew what they were signed up for, so would all of those other islands that got "punished" in that brief scene in this arc. Of course that scene made it seem like they're pillaging those islands, but remember Big Mom has always had a rhyme and reason for every action she does that makes some sort of logical sense, so I doubt those are just pillaging for the sake of pillaging.
As I said the Strawhats have wronged Big Mom much much more than Big Mom has wronged them. Let's look at what they've done...
- Ate all of Big Mom's candies which is produced to ease her eating disorder. (non-intentional)
- Sent her a bomb in a present. (non-intentional)
- Declared war against her and stole her territory of Fishman Island.
- Keeping Ceasar hostage despite Ceasar scamming her. When Big Mom got Ceasar back, she gave him a second chance KNOWING he's already screwed her over.
- Destroyed the Seducing Woods, defeated Cracker, defeated many soldiers in the Enraged Army despite the order of the later to capture them alive.
In comparison the only thing she's done is arrange a marriage between her daughter Pudding and Sanji, which was technically only invitational even though Capone forced Sanji to go with him.
Yes some of her children and subordinates are a bit twisted in the head, but I don't think that makes Big Mom overly evil. I really think this is a classic bait and switch situation kind of like how Franky was "supposed" to be the villain of Water 7 but it evolved into the CP9, which many people say this arc resembles.
