Why? On what evidence? Look at homicide rates across the country:

(
source)
While I'm sure @freddoso is a good and reputable man, that graph is sinfully uninformative (I honestly stay out of gun control stuff in general, but data I can't let slip by). Without looking up the laws of individual states, it's a bit difficult to know any effects. It didn't even label its vertical axis! Shame on you David!
SHAME!
Here's a chart that I think is more compelling, and more fleshed out with an ability to look at how laws differ across the country.
There's not a 1:1 relationship, but it does raise some questions, I believe.
It doesn't include D.C., which is an outlier as Yoda mentions, but indeed it is an de facto outlier based upon the fact that it's a metropolitan area rather than a state; most of the time when we consider state level data we don't include D.C. because it'd throw off a lot of calculations. Much more fair to compare it to metropolitan areas (which I'd be interested in seeing, I'm not actually sure how city level data bears out).
Though if you compare it to metropolitan areas, remember your lessons on causation; if we look at a city with a high crime rate, and a large police force, we should be careful not to assume it's the large police form that's
causing the crime.
Though even if you added D.C. to the bottom, I find the direction the data is pointing to be an interesting thought. (Possibly get rid of Hawaii and Alaska also, I would have kept it to contiguous states probably)
The argument ought to be: will it save lives? Will it protect people? If it would, then you at least have a difficult argument about what kinds of rights we're willing to trade away in the name of safety, and what kinds of costs various freedoms are supposed to have (and I'd really love to hear a gun control advocate at least admit there's some nuance to that question).
I've had the chance to work personally with the
Brady Campaign, and I can undoubtedly say that they have an incredibly nuanced view on the problem of gun violence, and are honestly a wonderful (and even pragmatic) bunch. Obviously, there are probably some people who have reductive views towards the issue, but I think you might be painting with too broad a brush.
I will also say that I've had the chance to work with the CA chapter of the NRA, and found their lobbyists similarly kind and straight forward (They are also a bit more nuanced than they seem, we even got to work together for funding on the dealer record of sale backlog). Sometimes that actions of the group itself have been a little...iffy..however. But I never had to interact with that aspect beyond their organized efforts to keep our phone's busy non-stop during the last gun control push around Sandy Hook.
Fun ideological exercise for my left-leaning friends: take any argument about gun control and replace "guns" with "drugs." Still like the sound of it?
Don't find this a particularly compelling thought experiment, I think it's a bit of equivocation. I could similarly say, replace "guns" with "grenade launchers". What we're talking about is a spectrum, and how far down that spectrum gun control should go, I
usually don't hear all or nothing stances.