Yeye,
Atheism as I understand it is not a derogatory term. It simply implies you do not belive in deities. It is only derogatory when used by people who share a common belief in a god. If you do not, it is a harmless label.
In its broad sense, paganism (also see pantheism), and animism are similar because of theior reverence for mother nature. In its narrow sense, it denotes a form of worship that exist outside christianity. In this sense, a muslim counts as a pagan to a christian because he rejects the trinity and Jeusus' supreme authority. I look at the narrow use of paganism in christiandom to label infidel as used by muslims on non muslims.
Paganist spirituality in nature puts them on a collision course with monotheistic cults like christianity and Islam. Indeed, the greatest deniers of environmentalism pollution and climate change are christians and muslims because they belive their god gave humankind the power to conqure the world. Those who believe in natural pattern of the universe are thus seen to negate gods mission to humanity.
In their quest to deligitimize anything that challenged their 'inspired' religions, the middle eastern religions attributed all the negative connotations and demonized the traditional form of worship, hence todays negative meaning of paganism. There is absolutely no problem with these terms if we can purify them and bring them back into the mainstream.
Emotions are the greatest enemy of rational arguments