The best explanation of this I've seen is in Lamp of Mahamudra by Tsele Natsok Rangdrol.
Basically, if we consider everything at the abstract organizational layer underlying both the mind and the matter, the so called "ground of all" -- we will see that everything is inherently perfect. It is the fundamental "suchness" of things as they are (whether experienced by someone or not). Because the universe is already how it is, for perfectly natural and objective reasons, everything that exists or does not exist exists or does not exist for a reason. In this sense everything is complete, without a flaw. That's Buddha nature.
But part of this Buddha nature is natural tendency of the mind to grasp or cling, or in modern terms, to create flawed static models of reality, that are more like caricatures than models, and then get stuck on them. This gives rise to such natural phenomena as ego, the three poisons, and suffering. These two are part of suchness, and in this sense they are also Buddha nature. However, what they do to us, is they make us confused and we start seeking happiness in wrong ways, and the more we seek it in wrong ways the more confusion and suffering we generate. All this time the Buddha-nature stays perfect because all this confusion and suffering is included in it. Buddha-nature or natural suchness of things however they are, accommodates everything -- that's why it's unconditional and indestructible.
From this perspective, whether you are confused or enlightened does not make any difference -- anyway you are included in the Totality of the Vast Expanse. But subjectively, from sentient being's perspective, being confused or enlightened is a difference between suffering and cessation of suffering, so in that sense it makes a huge difference.
Makes sense? Confusion and enlightenment (or contamination vs. purification) exists only in one relative sense but in the ultimate sense all of those are a natural play of phenomena subsumed in the Buddha nature.