I don't know, most people are almost never happy with what they have, no matter how rich their lifestyle is, and will desire for more.
I think if history proves anything, particularly post-cold war in the west, it's that an abundance of food, entertainment and general safety creates a passive, docile population that's reluctant to upset the status quo out of fear of losing a comfortable lifestyle. If all the parasites are former humans then unless desire for power is specifically selected for then most would likely seek minor improvements in their station and not go on a murderous rampage for power.
If the parasites get sick of the cheap, toxic junk food their hosts eat and they can't afford healthier, more luxurious food, they may take drastic measures to get the richer lifestyle they want.
A peaceful world doesn't mean the population is going to be feasting on mcdonalds every day and if the state of the world is as dire as it's portrayed then the diet of the nobility is probably not drastically different to that of an average health conscious upper middle class person. As for drastic measures, the MC has shown that the parasites can use brain chemistry to subtly affect their hosts behaviour towards desirable outcomes, the most likely scenario is a host that gets happy feelings every time they eat a salad and go for a run
Aside from Colon, these other parasites seem to not care about their hosts and are only looking out for themselves.
Being born into a desperate situation has that effect, it would be difficult for the parasites to feel much empathy for their hosts when they're at risk of death themselves. Colon may be different because he's a more moral person but if he was born into a starving orphan would he nobly perish or would he cling on despereratly, sacrifices be damned?
Oh I agree in that peace and coexistence is the most logical decision and the most beneficial and even profitable in the long run
But sadly, people are not logical beings.
Hate to bring in irl stuff, but humans with power fear losing power the most. And they will do all sorts of dumb, short-sighted fearful tactics just to keep power for one second longer
If these Parasites were cold alien beings, they might do the logical thing. But they seem to be isekaid humans. And humans are ever afraid of loosing stuff
Or maybe I'm just being a paranoid nyegative nonce, idk
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True, even in a paradise there are those who not only want more but want others to have less and if the parasites are all former humans then that would be as true for them as anyone else. Humans are afraid of losing, but someone with nothing to lose will take far bigger and more dangerous gambles than someone with a stable job, a nice house and a comfortable life.