Most religions have some kind of “golden rule” type of advice, suggesting that whatever you do in life somehow comes back to you. Some call it karma (though many westerners grossly misuse the word), and others simply have no name for it. When it comes to paganism, the general consensus lies somewhere between the Threefold Law and the overused phrase of “Harm none.” If you’re an atheist witch who chooses to mingle within the pagan community, you’re definitely going to run into masses of people who believe that every single pagan out there follows one of these karmic principles. And every pagan is going to differ on exactly what they consider “dark” or “light” in regards to magical practice. It gets tiresome, to be completely honest.

The Threefold Law came from fiction — and is widely misused!
Gerald Gardner wrote about it in a work of fiction, but many Wiccans (most, actually) tend to use the phrase in real life. Sometimes called the “law of three,” this idea came from the book High Magic’s Aid, and was pretty explicit in its meaning.

Mark well when thou receivest good, so equally art bound to return good threefold.

The quote pretty much implies that witches are supposed to return favors with something extra. If someone gifts a witch something, or does a favor that benefits him or her, then the witch (in this work of fiction) must then return that favor, threefold. On the flipside, it could very easily be insinuated that doing harm to a witch would also require that he or she return the favor, times three, as well. Knowing that this is the root of the so called “threefold law,” it’s worrisome to hear so many Wiccans (and other similar followers of pagan paths) use the term to imply that anything you do will somehow come back on you, much like the way karma is so often misused. 
Atheist witches aren’t bound by “The Rede”
I’ll be honest, I like the whole idea of “if it doesn’t hurt anybody, do what you want,” but I think that modern Wiccans have been taken over with the same overall mentality that plagues fundamentalist Christianity. “Do what tho wilt” could very easily be taken as “live and let live,” but if you step into any room full of pagans, or interact with them in groups on social media, you might find that some will try to shove the concept down your throat. If you want to talk about “dark” magic, or if you want to discuss sex or love spells, don’t be surprised if you’re bombarded with a slew of “harm none” comments. I think that many have lost sight of that advice, and choose to use it to bully others who do not recognize such ideas.
Atheist witches don’t recognize the Wiccan Rede. We don’t recognize the fictional “threefold law,” either. However, every atheist witch is different and may look at things entirely different in regards to “getting what you give.” For the most part, we don’t believe that some magical entity or force is hovering over us waiting to punish us for our wrongdoings, and we certainly don’t take kindly to others telling us that there is.
Are atheist witches good people? 
Every human being on this planet is different, and belief in deities or acknowledgment of threefold concepts doesn’t really change that. In general, atheists don’t need to be threatened with mystical punishments in order to be decent people, just as some heavily religious folks do indecent things even in the face of those same threats.