With empathy, it turns out that Germans were much more likely to empathize with other Germans than with Juden. With empathy, everyone was cheering as the witches burned.

This required first to, basically, decide that something which looks like a person is actually not and so is not worthy of empathy. That is not a trivial barrier to overcome. Without empathy to start with, burning witches is much easier.

Moral progress is the progress of knowledge.

This is a very... contentious statement. There are a lot of interesting implications.

All I'm saying is that whenever you have finally decided that you should make the world a better place, at that point emotional empathy is a bias that you should discard when choosing a course of action.

And that is what I'm strongly disagreeing with.

You are essentially saying that once you've decided on a course of action, you should turn yourself into a sociopath.