This is how I look at it.
If we were trying to prevent the number of people that die in car accidents, restricting the number of cars on the road would correlate to a decrease in accidents, regardless of whether "criminals" possessed illegal cars, or black-market cars hit the road.
This example is ludicrous because our society is so heavily dependent on cars. But our society does not depend on
guns. Restricting the number of guns in circulation, whether that be through banning automatic rifles, restriction of the number of firearms owned, etc., would, in theory, prevent death by guns.
Again, you could argue that you are only taking away the guns from the innocent, but regardless, tighter gun laws could have prevented situations like Columbine, Sandy Hook, Virginia Tech, etc.
Link to Harvard School of Public Health research article on the relationship of the amount of firearms to the amount of homicide.