Were the Littleton gunmen goths?

No. While black clothing and black nail polish may have made them appear gothic to outsiders, internally there was no debate. The gothic community overwhelmingly rejects these people, not just because of what they did, but because they don't even seem like goths. Their obsession with Nazism, guns and violence are clear indications that they did not subscribe to the gothic aesthetic. The gunmen's interest in industrial music suggests that they should be classified as "rivetheads," but saying that certainly isn't fair to people in the industrial scene.

Why not goth?

What separated goth from its death rock ancestor was a rejection of the anger in punk music. Goths are an inward-looking people, and as such, are non-violent by nature. To be fair, goths do not advocate peace; you'd have to have faith in humanity to do that. For whatever reasons, they simply live it.