How do you explain much lower homicide rates in different US states, many on par or better than "every other first wolrd country?

If you drill down deeper, by county level, or even more granular - the picture becomes much clearer still. You should research it, see what variables drive it, instead of arriving at conclusions without any data.

Why do such discrepancies exist, sometimes literally across the street?

The problem is much more complicated than "guns bad".