It's invalidating because it implies our Nparents actually tried their best. Or even tried at all.
Take this scenario from my own life- you are broke and have only $10 and with that $10 you can either buy bread and milk to feed your kids or buy cigarettes. For a parent that 'does their best', it's not even a question. You feed your children. You have a duty of care and they cannot care for themselves.
A Narc chooses the cigarettes every time, because THEIR needs come first. Then, the starving children get beaten for complaining about being hungry. Then, they are told to go to school and beg for food from friends. Later that day, they are made to scrounge the streets for cigarette butts to supplement the now depleted tobacco supply. Then made to beg strangers for money.
The children eat cereal with water for dinner and the Narc sits in their bedroom gorging on the supply of cream filled biscuits, lollies, chocolates and wafers kept in her room. One child literally faints and gets beaten for 'doing it for attention.' The children mix water and flour to make makeshift food and survive on that and condiments alone for about four days. The Enabler gets paid again the following wednesday and the Narc gets her multiple child support payments. The Narc ensures she gets her cigarettes, her magazines, her new shoes, her makeup, her clothes and jewellry, all the snacks and treats for her supply then buys less than $20 worth of groceries with the rest. Enabler works nights and sleeps all day but still sees it happening and does nothing. And the cycle repeats.
Exactly what part of this was 'doing their best'? The part where the Narc starved the kids? The part where the kids were beaten for starving? Forced to beg and scrounge? Left to be malnourished and sick? Not given medical care (in a country with universal healthcare, no less)? Or was it the part where the Narc made sure she was in luxury and the kids were practically destitute, Enabler doing nothing to help along the way.
No, Narcs do not 'do their best'. Telling the children of Narcs that their parents did their best is invalidating, because it completely ignores the selfish, spiteful, psychotic and purposefully hurtful things Narcs do to justify their existence.