Ashnard was king of Daein for 18 years being perfectly happy just ruling. Sephiran only told him of the medallion a couple years prior to PoR, more than a decade later. Begnion's normal citizens generally felt guilty of the massacre as well. It was literally only the corrupt senate that was the problem. He could've just attacked them, maybe send Zelgius at them after finding him. At the very least, he should've sent Ashnard after Begnion instead. But no, he goes into a Suicidal Cosmic Temper Tantrum and thinks everyone needs to die for this. If they wanted to make him crazy, at least present him as crazy. Instead, they try to portray him as sympathetic and an anti-villain who turns on Ashera even though he was the one who caused genocides to summon her in the first place. It's Fates level writing.
There is nothing Sephiran can do to stop the Begnion royalty from enslaving Laguz and otherwise staying at odds with the Laguz. Beorc and Laguz had already broken the vow renouncing war in the first place, so no matter what happened, Ashera was going to judge the world the exact same way she ends up doing in RD anyways. Lehran's entire job was to sing the Galdr of Release so that Ashera and Yune could talk before Ashera decides to pass her judgment, but he could no longer do it because he was stripped of his powers when he lay with Altina. He resided in Goldoa for 700 years watching the Laguz be enslaved and when he finally found his Granddaughter, she was promptly murdered for being Branded. Then his people were wiped out. He lost EVERYTHING.