Ouga would more or less be the phonetic spelling for ogre because of how they don't really have just an r sound in the language the same way English and many other languages do, if I'm not mistaken. So it was either supposed to be Ogre all along, but that can be hard to infer from how it was written in Japanese or it is supposed to be Ouga but for the same reason a misunderstanding has occurred and they think it's supposed to be Ogre even if Ouga is actually right.
This is the kind of thing you see come up in fan translations a lot because they simply don't get to ask to official source which one they meant, or because a source material (if there is one) doesn't have an official translation yet either and so you can't reference that for help in these cases.
INB4:
1. The MC does not consider reincarnators as "people", so it's okay to kill them, and:
2. The MC is a psycho, so murdering someone does not harbor malice. It's purely a neutral action, so the guy's ability does not trigger.