A-the-ism. "A", lack of/without. "The(o)", god. At it's core atheism means that you don't believe in god, any god. So no, believers can't be atheists because they believe in at least one god.
You are half right but I understand where you are coming from, you see extremism as what the bigots tell you it is (feminism, LGBT+, etc). But I in fact thing that taking an idea and turning the notch to a 100 always turns it bad, for example: Feminism turns into misandry when turn to the extreme, right wing turns into facism, black right movements can turn into black power, religion turns into cults, etc.
But I agree that there are some cases in where this does not apply like gender equality (but thought I don't know how that works tho).
What distro were you using? I had a problem with most distros of eventually dying on me until I installed Debian, which I have been using like a year without a problem
A-the-ism. "A", lack of/without. "The(o)", god. At it's core atheism means that you don't believe in god, any god. So no, believers can't be atheists because they believe in at least one god.