TL;DR: Lead levels and violent crime are incredibly strongly correlated. This is much higher than traditional correlations between violent crime based on demographic stats (living in a city, being black, or being a Southerner all increase your chances of both committing or being a victim of violent crime).
This holds true at the country level, the state level, the city level, and the neighborhood level, and the evidence is extremely strong.
So people don’t cause crime; lead causes crime.
It’s sort of like the idea of replacement level in major sports. The quality of play may go up or down over time, but we mostly judge people in relation to their peers and what we expect the average person would do.