peko

The root gives you the verb peki for free, so you rarely need fari pekon - li pekis does the job. Add -ul- and you have pekulo, a sinner; senpeka is sinless, and pekinta is the one who has already done it. The register is religious first, which is why the word turns up so often in the older literature Zamenhof translated, but Esperanto uses it lightly too, much as English does when someone calls a second helping of cake a sin. Note kulpo is the neighbouring word for fault or blame with no theology attached.