Again, gun-related homicides (and gun-related armed robberies) are the only crimes for which the US is a notable outlier among first world democracies.
The US is actually near the average of European countries on crime rates overall (remember that even in the US, homicides are a fraction of total crimes).
The most common form of violent crime is assault, and the US is below Sweden, England, Scotland, Ireland, and Australia in total assault rates and just above Canada, Germany, Finland, and Belgium (see top of page 36 in the linked UN report). For simple "robbery" involving "threat of harm" but not a gun, the US is below England, Belgium, Spain, and Portugul. IOW, the US is NOT notable for its violence or its poverty-driven crimes like robbery and theft. It is notable solely for crimes involving guns, and therefore for homicides which are usually committed with guns.
Except for the gun involved, every other political, social, and psychological factor that contributes to homicides also contributes to other violent crimes like assault and to non-violent crimes like theft. The "gun-related" part of these outlier crimes is the sole aspect of them that is not common to other crimes. Thus, it is the only possible factor that could be responsible for the US being an outlier on those crimes and not on the others.
The bottom line is that the total lack of controls on the legal gun market allows legal gun dealers and owners to profit off of illegally funneling guns into the black market with almost zero chance of being caught and minimal penalties even if they are caught.
The guarantees millions of guns flowing freely into violent criminal hands cheaply and easily. That guarantees much greater use of guns in situations that would otherwise just be a non-deadly verbal of physical confrontation.
Of course guns are not the root cause the motivations that drive the person to engage in some form of crime and confrontation to being with. But a gun is a neccessary cause for a gun-related homicide, and non-gun homicides are much more rare because they much more dangerous for the perp, and usually require more planning, skill, and time (which also means time to not go through with the act). Thus, availability of a gun is the most proximal and neccessary cause of the vast majority of homicides in the US.
In sum, it is not an open question. It is beyond a reasonable doubt that gun availability is the primary and most proximal cause of most homicides in the US. Remove that availability, and the other factors remain, but they would usually wind up resulting in non-lethal criminal acts or no crime at all (such as mere aggressive words).