Intelligence has no bearing on whether or not someone is nice. A person does not have to be smart to be nice.

How do you know that he has no idea that he is hurtful? If he says something rude, like 'you are fat, stop eating,' does anyone call him on it? Does he apologize? It seems like a really rare person who would not understand that when he does X and people say it bothers them, he needs to stop doing X.

If you want to hang around with him, that's fine. If you don't, that's fine, too. You are free to make a choice that is different from the choice the rest of your family makes. You can change your mind later on, too.

You are in a very awkward position. I have had similar feelings about my own father and go through periods of talking to him and other periods of cutting him off. It's really hard to know what the best thing to do is. Like you, I often find that I am motivated by feelings of sadness for his poor health (which in my father's case is caused by extremely poor decisions).

Any choice you make in this is going to have a downside. I would go with what works for you in the moment, as much as you can.