You replied that Stalinism couldn’t be worse than the Tsarist system.

Where did I claim this?

we mean compared to an easy-to-imagine, common-sense alternative for that time and place.

Why is this your comparison?? If the claim is that Stalin “showed up” and things got worse, it follows that your point of analysis would be comparing against whatever immediately preceded Stalin.

This is like if someone claims capitalism makes things worse

Actually, it’s not at all like this… a better comparison would be if someone said “capitalism showed up and made things worse”