From the Rounds' point of view, these Triangles are different people and pagans. With their belief of "Pagans are the devils", the Rounds attacked the Triangles at their first meeting, whom they misunderstood as devils.
They massacred the other ethnic group.
The Triangles have few weapons. They have never imagined that they would be suddenly attacked by perfect strangers. The Triangles were killed without any resistance.

Ref. Space Pagans / Nervous at First Meetings

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