In my experience it's more like "because I am uncomfortable with the idea that we have no purpose, there is no afterlife, and children die of cancer for no reason."
atheist is probably fine.
if you wanna be anal about it:
atheist - there is no god
agnostic - i don't know if there is a god and i don't care
Atheists don't claim that there are no gods. It's is impossible to prove the non-existence of something, so if you make such a claim, you get into belief territory. Most atheists are agnostic atheists, i.e. I'll be happy to believe in your gods once you present evidence for their existence.
Fwiw, the "a-" prefix (see also "an-", eg, "anhydrous") can be described as "without", "lack of", or "not".
Work out the definition for "theist" and go from there.
right but anti theists OPPOSE gods. which would lead one to presume that they might believe they exist.
Antitheists oppose religion, not gods. I'm an atheist because there's no reason to believe that anything supernatural exists, and I'm an antitheist because I think belief without evidence is inherently harmful.
Antitheists oppose theist thinking. The term is usually used by people who identify as actively opposed to religion rather than just nonbelievers.