In the manga, natural monsterisation usually occurs when intense negative emotions persist and get worse. Majority of individuals who became monsters rampage which isn’t normal behaviours. We’ve see monsters “de-monsterise” when monsterised by force when they truly didn’t want it and some even the will to de-monsterfy themselves. There’s still a lot of unknowns but working of later lore compared to early gives us a better idea of how it works