Looking back, I never quite bought the whole 'Voldemort is evil because he's afraid of death' thing everyone repeats. Sure, that's part of it, but that feels like a symptom, not the disease. His badness stems from a much uglier, more mundane root: a complete inability to see others as real.
He's not just selfish; he's solipsistic. Everyone around him is an object, a tool, or an obstacle. His followers are disposable pawns. His horcruxes aren't just about immortality; they're about making fragments of his own soul more important than whole, living people. He splits his soul to live forever, but the act of doing so requires him to treat murder as a mere mechanical step. That's the core of it—reducing human life to a means to an end.
You see it in how he interacts with even his inner circle. Lucius Malfoy fails, and he's humiliated. Snape asks him to spare Lily, and he sees it as a weird quirk to maybe indulge, not a profound love to respect. He doesn't understand love, loyalty, or sacrifice because those concepts require acknowledging that other beings have internal worlds as rich as your own. He literally cannot comprehend why Harry would walk to his death in the forest. To him, it's just a tactical blunder.
So his badness isn't a grand, theatrical evil. It's a cold, hollow, utilitarian emptiness. He's bad because he's less than human, not more. He lacks the very things that make the wizarding world worth saving, which is the whole point of the series' conflict.