I think it's impossible to say how much any factor contributes in a quantitative manner. "36% of crimes were committed in high lead areas, whereas 23% of crimes were committed in high crack consumption neighborhoods"?
I think it's all a mish mash of variables and it's best not to pick apart too many strands of data.
One thing we can say for certain is that murders are down 50% since 1980 despite population in the USA doubling. That is an amazing achievement and I highly, highly doubt it was chiefly due to our generation not having lead in their blood.