That doesn't explain the mass rampage murders, which have been all committed by wealthy or upper-middle class perpetrators. Plus, violent crime in America is decreasing even while poverty is increasing.
Violent crime is decreasing, period. It is spurious (if not intentionally misleading) to suggest that this correlates in a meaningful way with changes in poverty.
IIRC, Freakanomics had a section correlating legalized abortion with a drop in crime ~18 years later. Also plausible. But, of course, not necessarily causal.