My understanding from the intro is that he was adopted into the papacy and determined to be a magical prodigy (more in line with Roxy than Rudeus, but also without Roxy's decades of adventuring experience). So he's an adopted noble, and also directly in line to be the next pope or something. He's just an arrogant little shit as one would expect a magical prodigy to be. Rudy was exactly the same way before he got some similar doses of brutal reality.

The little girl and her knight are heirs to the rival faction also aiming to become the next pope. He was willing to watch because all the masked figures are on his side and from his family's forces. He couldn't be recognized by either side and he was actually in as much danger as the little girl was.

The issue is that his faction welcomes demonkin and non-humans, and the little girl and her knight they rescued are with a human supremacist faction.

So Eris actually did a bad thing.