From Wiki:
"Nihilism is a philosophical doctrine that suggests the lack of belief in one or more reputedly meaningful aspects of life. Most commonly, nihilism is presented in the form of existential nihilism, which argues that life is without objective meaning, purpose, or intrinsic value."
The nihilism you're writing about is a specific branch, and it states that life doesn't have an objective meaning or intrinsic value. From a scientific standpoint, this is true. Or, rather, that the genetic mission of mating can't possibly be the only reason to be alive. This allows individuals to decide on their own subjective meaning and choose their own missions and goals.
I believe our actions "don't matter" on a cosmic scale, but that doesn't mean I haphazardly behave immorally. Nobody that has arrived at nihilism through science could ever ignore that all actions have consequences, whether they matter or not. "Mattering" is a vague human sliding scale of importance.
I agree that an extreme nihilist might behave in a pathological fashion, but that seems to be true also for any person that takes an ideology to it's extremes. Every religion ever created has been used by somebody to rationalize something terrible.
I'm curious how many great and important social contributors in history had nihilist leanings.
Good point. I think that a lot of our development can be attributed to 'Nihilists' that refused to accept their position and 'meaning' in life, but once again it boils down to the responsibility of action.
Nihilism has a place in society just as pessimism does. To be human is to embrace each aspect of our psychology and learn to control it...