In this context, medieval times are "modern society". They already had social, religious and legal norms against killing. Also, most people lived in the country

In this context, medieval times are "modern society". They already had social, religious and legal norms against killing. Also, most people lived in the countryside or small villages where they didn't have many chances for killing people anyway, so the murder rate in larger towns must be a lot higher than the overall 35 figure.