Nope, not gonna teach you about Avada Kedavra today. This comes from thinking about living with yourself after you discover you're capable of throwing something like Crucio at another human being.
Yep, it's time for Adventures In Bad Ideas!
Only once in my magical career have I done "serious" cursework. A friend of mine had betrayed me very seriously, with a long and complicated series of lies. You're not interested in the details, but suffice to say I'd never been as hurt as I was when this unraveled. Moreover, I wasn't the only one who was hurt.
So I felt justified in the act. I was still transitioning out of chaos magic at the time, and I used a combination of techniques, tools, bodily fluids, symbols... Well, you get the idea. It probably didn't actually do anything, since the most effective curses require someone to know they're cursed.
I paid in my own time, but the worst part is that since then, every bad thing that happened to my former friend, I felt responsible for. Maybe this wouldn't happen to someone without my particular brain chemistry, maybe it was all projection, magical thinking and OCD on my part and his life fell apart on its own.
All I know is that I didn't do much beyond protection and the basics for years after that, and I've never wanted to do anything like that again. It's not that the curse itself is unforgivable; its not being able to forgive myself.