Homicide rates in the USA have varied a lot throughout our history... Roth has a very interesting idea about why.
After 25 years of research, he found that poverty, unemployment, discrimination, race, class, substance abuse, gun laws, and all the other factors we say cause murder actually don’t. Instead, as he explained to Zócalo, it’s about whether we trust the government, how close we feel to each other, and whether we think government and social hierarchies are legitimate.When people have high rates of trust in the government and the people in their community, homicide is low. When this breaks down, anger gets much worse, and some people lash out violently.
https://www.zocalopublicsquare.org/2009/11/01/randolph-roth-on-american-homicide/crime/Given where the USA is now, I am not optimistic. Consider George Floyd riots... presumably, lots of blacks decided that society and policing weren't legitimate, and responded with a huge spike in killings. I just hope it doesn't get worse.