I agree, it just Coppola that wanted to have him killed in the movie. But it wouldn't be realistic, mafia members (especially bosses) almost never resist arrest, as far as I know.

But maybe it would have been even more realistic to have him killed by some friends of all those guys he had killed at the end of part 3. I think that massacre he did (along with the massacre Michael did at the end of part 1) are 2 most unrealistic moments in the movies. No crime boss can be that powerful to go against so many different powerful enemies and win without being killed. Not even Carlo Gambino or Lucky Luciano could have attacked Vatican, Swiss bankers, Italian masons and rival mafiosi at the same time just like that without paying the price.