Some kind of dementia. It's a cancer of the soul which arbitrarily removes parts of you that you can't even perceive the loss of. I had an MS patient whose vocal processing was reduced to the point that he could only say variations of "you fucking bitch". He was totally bed-bound and dependent on a mostly female nursing staff for every single need. Most of those employees were burned out and he could only communicate to them using a wildly misogynistic slur. I've seen it reduce a famous AIDS researcher and a WW2 pilot to toddlers, others to cornered raccoons, for some it's a nightmare they can never wake up from and they just spend all day/night reliving their worst memories. For a good 10% on the ward it just takes away their executive function and they can no longer control their worst impulses or recognise that they should.

Cancer and strokes are a close second for more or less the same reason, but dementia is so existentially terrifying to me.