My MIL fits in here perfectly!
My DW and i have been together since we were 17 (well on way to 50 now), so i feel reasonably well-qualified to engage with the thread!
MIL has definitely experienced some severe traumas (unprocessed grief, in the main), following an extremely indulged childhood.
Outwardly she is a saint (heavily involved with the church, years working in special education), would give you her last penny.
Inwardly, she seems to have a complete inability to manage any sort of negative emotion. Fear, anger, sadness, disappointment, any sort of discomfort - everything is stored up, and then vented out at the easiest target she can find, as soon as possible.
Unfortunately this is generally my DW - given my FIL is entirely de-sensitised to it, and my BIL left home at 18, and maintains some extremely strong boundaries. Doing so outside the immediate family would also be catastrophic to her carefully curated public image.
DW was a much longed-for gift baby, and was very spoiled as a result. However, she is made constantly aware that this is the case, to the extent that she has always felt very much that she was the only reason MIL/FIL remained together (with the consequence that she was therefore at fault for how miserable both have been).
I'm sure MIL also suffers from depression, but again for reasons of appearance, would never ever seek any support.
Things have come to a head as a result of a DGC coming out - this has been a hand-grenade in MIL's life.
While she is capable of maintaining a (slightly chilly) relationship with the DGC, she refuses to even acknowledge the existence of DGC's partner - to the extent of leaving the room to avoid saying hello or goodbye.
This has been the final straw for DW - who was fine dealing with the crap as long as it was only directed at her.
MIL now has no-one to whom she can spill out all the bile she's storing up about this (and the many consequences of her very obvious behaviour).
She is absolutely unable to listen to reason, nor show any willingness to take responsibility for the consequences of her actions. So the discomfort, anger and sadness is just continuing to grow.
I've begged DW to get herself counselling, but she's not ready yet. So on we go, half tip-toeing round, half brazenly supporting DGC, not getting anywhere.