maybe I probably killed him
You didn't kill him, the cancer did. You were there supporting him.
I was on morphine duty when my grandpa died two years ago and it was rough. I gained a massive respect for hospice workers and the mental fortitude they have in order to work in that environment.
My grandpa had ptsd nightmares from the Korean War his whole life and my biggest fear was that while he was dying he'd be trapped in one of those. I drove myself crazy trying to interpret his twitches and mumbles as some sort of actionable message he was sending.
I stepped outside for a bit and imagined what he would say if he were conscious, and I'm sure it would've been something like "Who cares if I'm having a nightmare? Even if I am it's not your responsibility to fix it. I'll just ride it out and it'll be over." And so I went back in and just sat there quietly, occasionally making remarks or observations, talking about what I might be doing in the next few weeks or months.
I'm really glad your mom got to have that moment. Whether or not he heard her and was responding, I hope the timing gave her some peace.
Good question. I don't think it's actually funny so I don't have an answer for that. I agree with your take.