An interesting discussion.
I agree with everyone who has said that the anti gun folks are going to be antigun regardless of the gun's appearance. Gun haters hate guns, plain and simple.
There have been a few legitimate points brought up that the skull has been an iconic human image for a very long time and can mean a lot of different things to different people. I personally have a number of animal skulls sitting around in my home, outside in the yard, and on my desk at work (I'm a forester so it's acceptable in my office.) I've always loved the natural history museum feel of having different biological specimens sitting around. To me, there is nothing grim or dark about it. Death is a part of life. It just is.
If you really wanted to get crazy and goth with it, design some furniture that looks like arms and skeletal hands wrapping around the barrel, or holding out a bayonet. Then we are talking some serious Army of Darkness paraphernalia!
I have to acknowledge the creativity of it also, and similar items could be made with different imagery.
I guess I don't particularly like it, and would never put something like it on one of my guns. As some have pointed out, if you did use that gun for self defense, I can definitely see a lawyer trying to use it as an indication that you are a nut or a vigilante. In that context, it could kind of be twisted against us. Otherwise, it's just a gaudy accessory.
My approach to society is to be peaceable, but with just a hint of menace. I'm talking about manner, not decoration.
My approach is to simply avoid society. "What's that? The group is headed that way? Ok, well....... I'll be over here........ or just out of sight."