There is a lot I don't know about this, but I'm sure you're right that the salt taxes cost lives. But at independence the taxes had been deeply unpopular for more than a hundred years, so it's not clear to me whether to credit Gandhi for ending them. I had thought that it would be interesting to know whether they had been phased out in Pakistan as well, where Gandhi is not so popular, but it seems that Nehru ended them a few months before partition.