Correlation doesn't equal causation. In fact, i'll hedge that these are barely related at all. Let's start with the crime stats.
To begin, Violent Crime is an enormous category that covers far more than just guns. That being said ... policing, as a whole, has greatly changed since 1991. Take a criminal justice course and witness this. Start with better police tactics in most cities as well as younger (and bigger) police forces, particularly after 9/11. Dramatically different and more extensive use of surveillance and other technology in policing, which is making it easier to catch and convict people after a crime is committed. Also look at the skyrocketing rate of incarceration (for most crimes) since the 70's thanks to private prisons and a prison-industrial complex that believes longer and harsher sentences are the way to go. Not that these sentences are necessarily deterring crime (they aren't) but that the people who do commit crimes are more likely to get caught and significantly more likely to spend more time in jail.
Add to that the fact that drug use as a whole (excluding marijuana) has either plateau'd or decreased since the 1990s. Would you agree that less "hard" drugs = the potential for less crime? I would.
I don't think the skyrocketing rates of gun ownership have much, if anything to do with a decrease in crime rates. I say this because the majority of the registered guns in this country belong to a minority of people. The united states is living a weird double-life where the number of people who own guns is diminishing, but of those who do own guns, the average number they own is climbing. Meaning you have gun owners stockpiling weapons and ammunition, not a country arming itself.
Many of these people live in states like Kentucky, Montana, Wyoming, and Alaska. Not the most dangerous states in the country. Most guns are going to these states. Meaning that relaxing gun laws won't necessarily start arming the country, but it will appeal to people who spend money on guns because they can buy new toys.
Relaxing gun laws isn't going to help this problem.