Since, the 1990s the homicide rate in America has come down considerably from the much more violent 1970s through 1990s and while there are a lot of theories about why this is so, it is so. At the same time, aggravated assault, has followed the same trajectory of peaking in the 1990s and also has steadily declined since then. The reason that the aggravated assault category of crime is so important, is that because of medical technology, emergency rooms, hospitals and their medical staff, it is more likely that people suffering from crippling wounds can survive them today than they would have back in the 1990s. Additionally, murder is the type of category of crime which is fairly straightforward to categorized, since the victim is deceased, and the determination of foul play is usually self-evident. On the other hand, aggravated assault is far more nebulous, that is to say, prosecutors may easily plea down what appears to be attempted murder to felony aggravated assault just to assure themselves of the conviction, as well as the fact that attempted murder is far trickier to convict a perpetrator of, because it is somewhat problematic depending upon a whole slew of varying circumstances needed to coalesce in order to prove the "intent" of the perpetrator.
In any event, both homicide and aggravated assault have come down at comparable rates over the last 25-odd years, which would imply strongly that there isn't less murder in America, because doctors have all of a sudden, done far more adroit work in keeping victims of violent crime alive, but pretty much is a reflection that less people are being targeted to be killed by criminals in the first place. That truly is good news, because far too often, statistics can be manipulated in a manner so that one thing is overemphasized or distorted to get across some subjective point.
Of course, the other consideration to take account of, is why have those homicide and aggravated assault rates come down substantially over the last two decades, in which conservatives often want to point out it must be because of our police and prosecution forces combining together to punish and thereby take dangerous criminals off of the streets and incarcerate them, which America has performed at the highest levels in the western world. There is though, a very strong case that in recognition that the demographics have changed in America over recent decades, in which statista.com shows that it is the age group of 20-24 which represents the highest murder offender percentage rate in America that has brought down violent crime statistics, because the median age in America has increased from 32.9 in 1990 to 37.8 in 2015, signifying that an aging population will have less homicide and aggravated assault crime. In short, probably the combination of favorable demographics along with more sophisticated and dedicated policing tools have together combined to reduce violent crime, with contributions from a better managed social safety net helping to lead to less violent crime.
In any event, there is indeed less violent crime in America, probably not because crime doesn't pay, and probably not because people overall are more considerate and caring than they were before, but probably because higher incarceration rates in conjunction with favorable demographics as well as effective social services have reduced violent crime.