Ashnard had already used the Blood Pact to kill his family until he was the one in line to become heir to the throne. This is around the time Sephiran's granddaughter is murdered. There was going to be a war regardless of what Sephiran does at this point. So he tells Ashnard about the Medallion.
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.