To be fair, the crystal ponies treated him like shit, and the princess let him suffer for years both from the crystal heart, and living in paranoia of becoming a monster, which his mother basically confirmed for him.



The guy went through hell and eventually snapped.



Technically he already got punished for what he did (got sealed sway for a millennia, then nearly destroyed body and soul.) True he didn't get punished traditionally, but he gets to look for thousands of fragments of a petrified pony, which doesn't sound too weak of a punishment.