If Chaos is so benevolent, why do did the people of Gereon fought back? Because they were horrible. That's really it.
"If the Imperium is so benevolent, why do Chaos cults rise up to challenge it on planet after planet? Because it's horrible. That's really it." Do y'ken why this line of argument falls flat on its face with a resounding thud? Not to mention you still kinda failed to address the whole bit where the Inquisition decided to reward those who resisted madness and mutation by herding them into camps to be experimented on. (See also Armageddon, First War Of, when the Inquisition murdered more people cleaning up afterwards than Daemon Prince Angron and the World Eaters did in the war itself)
And hey, maybe the fact that the people from birth they were indoctrinated into an all pervasive religion that teachs that hatred of anything except the Imperium of Man is a holy virtue and the most noble death is to die killing an alien, mutant, human secessionist etc,
maybe that might have had something to do with why they chose to wage an insurgency against the occupying Chaos forces. Possibly.
Not the sole reason, you're right, the Chaos forces stripped the planet clean and used the population as slave labor to feed their war machine, that breeds backlash and resistance.
It is also not noticeably worse in degree or kind from the things the Administratum, the Adeptus Mechanicus, and Rogue Trader dynasties do to feed the Imperium's war machine or line their own pockets. Yes, Ultramar's 500 Worlds are better than living on worlds like the Khornate planet Justicar Alaric of the Gray Knights was stranded on, or the various worlds in the Eye described in the Black Legion novels or the Word Bearer trilogy.
Counterpoint: said societies are not noticeably worse than being middle to lower class on Necromunda or any other Hive World, or being a menial on a Forge World, and are arguably nicer places to live than say, Krieg.
Ultramar is the exception and nowhere close to the rule, and all those unnamed 21st century Earth planets who don't have the most politically influential First Founding Chapter watching over them are still subject to having their entire population tortured to death if an Inquisitor gets a wild hair up their arse, which really undercuts your "nice as 21st century Earth" narrative.