I’m very sorry for your loss and I’m very, very sorry for how you found out about it. Posting on FB was inappropriate and cruel for the man’s family and real friends. From your description, I suspect Bill is clueless, not intentionally mean. But that doesn’t excuse his behavior.
It isn’t wrong to want him out of your life but it may be impractical. You travel in the same circles and your fiance considers him a good friend. Don’t put your fiance in the position of having to choose between you. He has a right to draw his own conclusions. But you have a right to tell Bill how hurtful his posting was and to take some distance. Ideally, you and your fiance should find way to limit your contact with Bill without putting too much stress on your relationship.
I wish you well.