I don't think it's an absence of belief in a god that causes militancy in some. I think its a recognition that others are different and therefore a threat. A 'belief' or fear of others taking over. Whether the others are roma, muslim, gay whatever there are people happy to identify them as different and therefore a threat and therefore legitimate targets.
I agree, I've never heard of people militantly proclaiming their violent atheism. But then I've never looked for any evidence of that either, something in the back of my mind tells me there was a nazi campaign along the lines of 'you are either a christian or a nazi', but that's hazy so I won't use it as an example. One for looking up later. But by the same token, plenty of non-religious people have killed the religious because those people have self identified as different or non-compliant in some way. From all the usual examples that are wheeled out, Stalin, Pol Pot etc. Not religious themselves but happy to kill others that had a belief in a different system to the one they were peddling at the time.
But then, my own personal 'belief' is that of all these people that kill in the name of their god, the majority don't really believe in it anyway. It's a prop, a gang, a tribe, a legitimacy.