I have never personally understood dualism very well, although it seems to run rampant in many corners of the pagan community. In my particular focus on fairies and fairylore I also see this dualism expressed in the idea that some people have that fairies are either good or bad, or in some views must be wholly good, or in others wholly bad. I tend to reject these concepts but I think its important to discuss why.

To begin let's start with what dualism is, so we're all on the same page, because there are several definitions. For our purposes here the meaning we are using is that of a worldview that divides things into the opposing groups of good and evil. In cosmology this is often expressed through the idea of powerful benevolent deity/deities in opposition to malevolent cthonic or entropic forces. The benevolent forces seek to preserve or improve the human world while the opposite forces seek to destroy it; that which seeks to preserve is labeled by humans as 'good' while that which seeks to destroy is called 'evil'. 

This philosophy is applied to fairies in several ways. The most common is in the hard division of all fairies into so-called 'good fairies' and 'bad fairies' or sometimes good and evil fairies. Many people lean into a modern understanding of the Scottish Seelie and Unseelie courts or the popculture Summer and Winter Courts for these classifications, arguing that there the Seelie are the good and the Unseelie the bad, and seeing them like two sides to one coin. In contrast I have seen people suggest that fairies are aligned wholly on the side of goodness and are unable to do anything evil; or else are wholly evil and unable to do any goodness. These concepts, any iteration of them, play nicely into dualistic thought but do not mesh well with the actual folklore.

I mentioned above that its become common to view fairies as good or bad because of the Scottish court system, although this is an oversimplification and partial misunderstanding of the terms unseelie and seelie. Because most people using the terms in the US don't have any Scots the words have lost their nuances, which is unfortunate because the nuances are important. Seelie doesn't simply equal good and Unseelie doesn't equal bad, rather Seelie has several meanings that include lucky, blessed, and fortunate while unseelie can mean unlucky, unholy, mischievous. Neither exactly means good or evil which is vital to understanding fairies. Also the terms are not universal but began as very regionally specific - in most places with fairy belief there has no clear distinction between the helpful and harmful fairies. 

It's also important to note that the categories of seelie and unseelie are not hard and fast but fluid. A being considered seelie by many accounts can shift into a more unseelie view if angered or offended. Brownies are a good example of that; if treated well and respected they are helpful around a home or farmyard but if offended or insulted they wreak havoc and destroy crops and property. Of course there are those beings who are more purely aligned against humans or see them as a food source and there are those who are more genuinely helpful and disinclined to cause harm. However even among these more set groups there can be a surprising flexibility - for example kelpies and redcaps have been known to aid humans (although in fairness the redcap was helping a really evil human, still).

Fairies in folklore do not easily fall into clear categories. We see them acting in various ways that range from malicious to benevolent and can change depending on our actions. In one Welsh account a young boy was taken into Fairy to be a friend to a young fairy prince; he was treated well until he tried to leave to go home to his mother, at which point he was chased out with threats of violence. Similarly there's an Irish anecdotal account of a musician who was invited to play an athletic game with the fairies, and they treated him well until he was offered food which he refused and a fairy woman put his eye out. There is a Scottish story of a brownie who protected a farm and always dealt well with the humans who lived there until someone tried to forcibly baptize him, at which point he disappeared and abandoned the place. We see accounts like that across folklore, where how a human chooses to respond to or treat a fairy directly relates to how the fairy then treats the human.  

To understand fairies outside this duality means accepting that the dualism is false. Ultimately the best way to understand fairies is relationally. Like anything in this human world even the more dangerous things are not evil, per se, but rather are simply true to their own nature and inclinations whether we are talking about plants, animals, or bacteria - or other people. Some fairies, like apex predators or poisonous plants, should be treated with caution, others can be dealt with on their own terms, respectfully, and others are reasonably safe, although it's always worth remembering that almost anything can kill you under specific circumstances. Water is a good analogy here - water is necessary for life but too much is deadly and bodies of water need to be respected or you will come to grief.