In other words, to paraphrase into modern lingo the Alter Rebbe’s concept (from Kabbalah) that three realms exist – the realm of good / holiness, the realm of opaque light and the realm of complete evil, darkness

in simple words, a human being has moments of unconditional love, goodness, kindness – but most of our moments are opaque light, for our interactions are not really – even when we love – done selflessly but selfishly

And unfortunately we have moments of pure unadulterated evil, vengeance (yes the fact you think it is right to do onto others as they have done onto you …) anger, and its worst form maliciousness, born from jealousy: anti-Semitism, terrorism, bullying (where it is not even vengeance, it is an outright harming of someone who never harmed another)
Not to be graphic but as it serves as such a useful linguist analogy:

You can either make-love, or a person can simply be using another for their own means, or they can be screwing the other (of-course this is exactly how all human behavior, all human interactions are.)