Atheism is all about how a deity cannot exist and anyone who believes so is a pitiable soul wasting their life believing in giant bearded wizards or some crap. It's far more venomous towards religious groups because they don't believe what they believe and they feel constantly oppressed by the religious imagery being shoved down their throats.
atheism isn't about any of that. there's a specific subgroup like that but they're usually angry people anyway from what I've seen. there is no good descriptor for an atheist because the only thing it identifies is whether you believe there's a god or not.
"I don't believe in a god and I don't care if anyone else does" describes most I've interacted with. Not surprisingly, they're laid back on most other stuff too. They don't make atheism their personal identity or feel the need to publicly crusade against religion.
Of course I've met people like you described too. Dudes who randomly interject into your conversation with "there is no God, actually" or whatever. Those dudes suck. They're a loud minority AFAIK.