I love a good villain. That character who terrorizes one or all of the other characters in a story. I love their backstories and I love what we create for their psyches. And then I love to see them get taken out by the good guys or by their own hand. What I don't love is seeing the softer side of a villain.
I have discussed this topic with a few writers and almost no one agrees with me, which is fine. I think we try to humanize villains to help make the world in our stories not such an awful place. Now, every villain doesn't have to be the most awful person we have ever read about, but I also don't want to see a villain who we are supposed to feel sorry for. I've been told that I have to make the villain have some redeemable qualities so the reader can connect with them. I say hell no to that! If my villain is truly horrible, I want my readers to hate my villain's guts as much as the other characters do and I do.
When we talk about a villain who is a teenager, we may not have to go so hard, and you can of course give your villain a softer side if you so wish. But, if you had that instinct to make your villain a true monster, you were not wrong in thinking that. Villains are supposed to be terrible. We don't need to like them. It is helpful to understand how a villain came to be a villain, but I do not think we should sympathize with their past to justify their awfulness.