My (speculative) take:
Suville troops attacked Tara because Suville had attack orders, it was a neutral region that Tara was defending, and Suville and Tara had neutral relations.
CE only ended up in the middle of it because of the federation. If it was only an alliance, I think they would have sat it out like Coria and Strombran did. Basically the battle code read it as Tara was attacked, looked at all other relationships and decided Strombran and Coria had equal relations with both parties, then looked at Ce and decided it had stronger relations with Tara than Suville and would side with Tara.
I don't think murderous matters here because Suville didn't attack CE or any other allied party. They attacked the Tarans with whom they are neutral. Then CE came to the Tarans' defense. It seems logical enough to me. If it's a bug, it's not a glaring one. To me, this could conceivably be working as designed.
*Edit: Corrected a typo
