My paternal GM was a narcissistic controlling bully, who would brag about how nasty she could be to her own family.
My MIL is a narcissistic controlling bully who believes she can do no wrong because she is a christian.
My maternal GM always had time for us, little activities, playing with us, lots of happy memories. Heavily christian, but without the bigoted comments to other lack of/faiths.
My Mum, as far as I can see much like my maternal GM.
They both bully GC from toddler age up, 'You're fat and spotty' (to a 13yr old), 'You talk funny' all weekend (you have a different accent to me)
They both start a conversation with their GC befor swiftly turning their back to do something else. I have clear memories of hurt when GM kept doing this to me though out my life, watching MIL do it breaks my heart for my DC.
They both love a huge party where they can show off their GC despite barely knowing the GC. Rather than ever wanting to spend some quality time with their GC, but nobody would get to see how much they love their GC!
Both are horrible gossips who always lie put their own spin on things.
They both, and this is very important, brought their DC up to worship them above all else. Breaking a lifelong deep belief takes something quite earth moving.
If I had realised history was going to repeat itself I would have run a mile. Since my GM died I've never cried for her, I remember what she did to us first hand.
My Mum struggled with MIL because she had been brought up in a family that respected and loved each other. There was no battle for family control in her young life. In the late 60's you blindly 'respected your elders' so when her new MIL started bullying her and later her DC (me) she could not cause a 'scene'.
I have struggled less. MIL's behavior is normal to me, though wrong. She lost all the respect I had for her years ago.
Dad finally woke up after 20 years and we went LC for years.
OH comes and complains to me about our DC being bullied but says nothing to his M. Then complains when I object!
They have both been conditioned to put their DM above all else and excuse here behaviour. They are not being lazy, they are trying to come to terms with their own mother falling very,very far off that pedestal.
The males I do blame are the husbands. I watch as my FIL rolls his eyes at MIL but says nothing. They are these womens enablers, they had the power to stop this, they choose the lazy path.
I think the we are having bigger problems now as previous generations had much more social constraints (and geographical, 2 hour drive to IL's 
)
that meant they had to put up and shut up, that's if they even considered complaining. Society has changed. I even feel sorry for MIL sometimes. She was telling me how her Mother and Aunt had discussed her career and had decided what she would do, MIL was married with DC at this point. She looked so sad. A few months later she tried telling me how my career would go after my DC 
I think MIL took all this shit for many years thinking she would have her time in later years, I've been a bit of a shock for her, but not her other DIL. So clearly I'm an evil DIL if only I'd actually marry her son, the shame .
There will and always have been nasty DIL's. But this is Mumsnet, not the demographic for DIL threads. There will be nasty people, so nasty friends/partners/mothers/IL's, the same person, different relationships. We don't suddenly turn nasty, a caring thoughtful person will remain the same.
My dc are young, they do the 'Mummy I love you the most'. As the conversion moves on to 'I will live with you forever Mummy' type thing I tell them one day they might meet somebody, like I met OH, and they will love them and want to live with them more, and to wait and see. I'm not putting myself on a pedestal.