What is there not to understand? “Love” (which is highly debated as to what it actually is, and realistically, I see it as either simply desire for social interaction + sexual drives, infatuation, benevolence out of a sense of moral obligation, or some other form of conditional attraction) is generally seen as a positive quality. Love is not malicious, at least not intent‐wise. A purely evil villain by definition cannot have positive qualities or benevolent thoughts, because then their evil would not be pure. Their malice is limitless, indiscriminate. If they’re as evil as can be, then why would they care if, say, something happened to their mother or even their spouse? Can you imagine Morgoth loving someone? Furthermore, would it make sense for a purely good hero to feel hatred for someone?
Not everyone can feel love, or wants to. Those with antisocial personality disorder (psychopathy) lack empathy and may not even value the lives around themselves. Nihilists, for instance, also may not value life (I am aware that this is not an inherent trait in nihilists—but as far as I know, the philosophy of nihilism, in a broad sense, rejects the notion of any intrinsic or objective value in life). A purely evil villain could have their own twisted form of “love”, but it would likely disregard the comfort and/or safety of its recipient, and it certainly wouldn’t be unconditional, nor love in a traditional sense; said villain would only value that person or thing for selfish, malicious, or simply pragmatic purposes. It would be obsession at best.
You can’t have a “former PE villain”; PE villains stay PE. If they turn out to not be PE, they were never truly PE to begin with, because PE villains are utterly irredeemable. That’s the whole idea. You can’t use love to humanize a character that is meant to be inhumanly evil, even if they are still technically human.
What good and evil objectively constitute is far too much of a gray area since morality is an ever‐evolving social construct, but by most people’s standards, I’d say these criteria make sense.