All the factors you mention contribute to crime, murder, suicides etc. They all should be addressed and to some degree they all are. Our prisons are full. We can do more. Lots more in some cases such as mental health. Obviously we can't eliminate human behaviours which create an atmosphere of crime, murder, suicide and accidents, but we can do more to reduce their frequency.
When you say that murder rates are decreasing do that mean that the more guns there are the less the rate of incidents? If it's true that rates are decreasing, for how long has it been happening and is it part of long term trend or just a matter of normal frequency variation? There are 300,000,000 guns out there. If there were only 1,000,000 would the rates be different? I think the rates would be much lower. The rates would increase with the increased availability of guns, so at 50,000,000 the rates would be higher one would think. At 150,000,000 higher still. The absolute number would be higher also because during that period of gun growth the population would have increased.
Now we have 300,000,000 guns and growing. Somewhere in those numbers there must be a point where more guns results in lower rates of crime, murder, suicide if there is a causal relationship. The increasing trend must have reversed from a positive one to a negative one. Is that what you are claiming?