One idea I've heard (but doesn't seem widely accepted, so take it with a grain of salt) is that a lot of cities had high immigration from the countryside but negative population growth from all other factors. Cramped conditions, limited food imports, limited potable water, and a lack of organized methods for waste removal tend to have some severe effects on population growth.

Killing a few people who were living the countryside and left because the family farm can no longer support them might not have any noticeable effect on the population. Might not even impact the death rate, depending on who is being killed and how.