I have an enemy. I define that as someone I would go (and have gone) out of my way to harm, but not significantly out of my way, nor harm to the point that legal action could be taken against me.

In my case it's an ex-coworker. He's directly responsible for the worst period of my life. The twisted part is that he has no idea how I feel, which makes retribution more satisfying for me; he has no idea where it's coming from. Only my wife knows how I feel about him.

Generally I don't even think about the fact he exists. It's more of a background thing, when the opportunity presents itself (once every few years), I act.

Is it healthy? Probably not. But it's also not like I spend my time designing schemes to get back at him. Is it normal? Also probably not. But I kinda like having this secret.