So this really happened. What can I do now?

I researched a keyword for a cats niche client and wrote a 1500 words article for him. Had found him on some other site (not platform). He paid for some earlier keyword research I did. Then he asked me to write another one. I asked him to pay for the first one first. You all have probably guessed what happened. Yes, he didn't pay me for the article. Now I checked his site on SEMRush and that keyword which I targeted in that article and yes, it is ranking.

Now, I am feeling really fucking stupid. I spent fucking 4 hours on that article. I don't want to sound racist or anything but this isn't done. This isn't done. He was from Belgium. If you get some work done and don't pay for it, you are a sick person. Seriously sick. And that is all gentlemen. That is all. Never deal with people outside of platforms or BHW on some other site. You are going to get fucking scammed. That is it.

I want to get back at him. I really want to. Can someone suggest something please? Something ethical and not negative shit like -ve SEO? Its not like I am lacking clients. I just want an eye for an eye.