This is at least my third try at finding the right words...
I can't imagine the pain you and your family are going through. You've been a great son by simply including your father in your life and giving him time together; it's horrible that time has been cut short. Please don't blame yourself for this accident; you had the best intentions in the world, and #### happened. Try your best to stop Monday morning quarterbacking it.
I know nothing about your dad other than what you've said in the original post; but it seems pretty clear to me he'd rather have gone out yesterday with a saw in hand than twenty years from now lingering in a nursing home with his mind completely shot.
A dear man I knew briefly went out quickly a couple years ago at age 75. His memory was starting to slip, and he'd always been athletic, but his body wasn't quite doing what he wanted any more. One morning he tripped going down the stairs for breakfast, and that was it; life flighted to the hospital, and died a few hours later. And even though I'd love to have had time to get to know him better, his time came and I have my memories.
You're hurting more than I can imagine right now; definitely get together with a friend, minister, rabbi, doctor, shrink, counselor or whoever you're comfortable with and make sure your head is OK. Don't bottle things up TOO much (hey, we're men, but sometimes you just have to let loose with whatever emotions are messing with your head). No shame in asking for help on this one. No shame in going on a minor bender, either, just have some backup in your corner to keep it from getting too bad.
Even though we haven't met, you have friends here. We're all thinking of you and your family.