You said violence is increasing, which is fear-mongering and needs citation.
> Never before in human history have humans lived in densities that they live in modern cities
Citation badly needed. We're in a crisis of low density in almost every urban area on the planet thanks to the auto industry and zoning laws, which I'll explain below.
> I’d want to see evidence that “It’s illegal to build walkable areas in most of the US”.
Ah, so the problem here is that you're interpreting the existence of sidewalks as walkability. That's not what it means: https://en.wikipedia.org/wiki/Walkability.
Walkability is a complex concept dictated largely by zoning, the practice legally defining the type and qualities of structures that can be built. And zoning, almost everywhere in the US, prohibits walkable urban design: https://duckduckgo.com/?q=walkable+neighborhoods+illegal+due...