These are pretty weak correlations. Gun control has passed all sorts of other places, which looks like gun confiscation, without mass slaughter.

Also, gun confiscation was not at all the defining characteristic of these other places that experienced mass slaughter. There are many more things that accompany that type of political landscape, and are more common.

And without an example of where citizen gun ownership prevented a dictator from coming to power, the whole argument is extremely weak, though it definitely gets repeated quite a bit.