Darkworkers can be parasites but they don't need to be, counter to the opinions of most on these forums. Your path can be entirely self-focused without inflicting harm on anyone you don't mean to hurt. Indeed, you can even experience love and empathy, you just won't do it in the context of oneness.
That's the primary difference, perhaps the dividing line: do you believe it is best to work on behalf of the whole, to identify yourself more with the whole than you do as an individual, or do you value your individuality above everything else? Lightworkers can be individualistic but they cannot separate from the whole. Darkworkers have no inborn responsibilities aside from their own needs. They must determine how they will treat others on their own. Most would choose a form of ethical behavior because it feels better than psychopathy. (Not to mention there is no direct benefit to being a parasite and dampening others awareness.)
I've outlined my views on polarity
here. If you're serious, I'd suggest running google searches on The Temple of Set, Polarity Magic, The Kybalion, Satanism (add in "love" and "compassion" to get some interesting articles), and The Left Hand Path. All of those are spiritual in their connotations barring Satanism, however they provide valuable insight into how one can walk this road without sacrificing wisdom for power. If you're completely allergic to anything spiritual, just think of any references to magic and what have you as relating to a part of yourself, primarily the subconscious.
I'd also recommend
The Darkworker Experiment. The site owner hasn't updated it in quite some time but what's there is excellent. It's mostly written from a neutral perspective so you can benefit from it regardless of what you believe about spiritual matters.