There are a couple parts of that explanation that sound logical and forthright by the way he'd been raised. Of course if these gems held part of someone's father and were made by him then they should, by right, belong to the clan of the father. If this enemy took the gems by killing someone then that person's children had a right to take them back and also seek vengeance on the enemy.

The part that is less clear, and given by how Maglor is reacting was the crux of his guilt in this story, is just who else he had to fight to reclaim the gems who weren't the enemy. It was like killing the whole Wen clan even if the peasants had no say in what their leaders did.

"Have the gods granted you peace now that you're here? A second chance, perhaps?" He considers the situation a moment longer before offering. "I can play for you and see if I can help your heart." He isn't the strongest of the guqin players in his clan, but he's been trying to learn the Song of Clarity. It won't be as effective as his elders but they weren't here and he was.