The details of my life are quite inconsequential ... Very well, where do I begin? My father was a relentlessly self-improving boulangerie owner from Belgium with low-grade narcolepsy and a penchant for ❤❤❤❤❤❤❤. My mother was a 15-year-old French prostitute named Chloé with webbed feet. My father would womanize; he would drink; he would make outrageous claims like he invented the question mark. Sometimes, he would accuse chestnuts of being lazy. The sort of general malaise that only the genius possess and the insane lament ... My childhood was typical: summers in Rangoon ... luge lessons ... In the spring, we'd make meat helmets ... When I was insolent I was placed in a burlap bag and beaten with reeds — pretty standard, really. At the age of 12, I received my first scribe. At the age of 14, a Zoroastrian named Vilmer ritualistically shaved my testicles. There really is nothing like a shorn scrotum — it's breathtaking ... I suggest you try it
It would be "Den onde" also, to mean "the evil one".
I never thought about adjectives turning to nouns by adding the definitive article, but I suppose you are right? You do in most cases have to add that "e" at the end though, or doubling of a consonant plus the "e", e.g. the color green is "grøn" but "the green" would be "det grønne". Seems to me it would be akin to adding "The" in front of words in English.
Technically "ond" is "evil" - whereas "mad/angry" would be something like "gal/vred".
I'd suggest that a word that might work better, being a little less biblical, would be "mean" (Why are you [so] mean!?). Another could be "malicious" depending on the context and setting? :-)