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.
Ashnard was a mad man. Tanath says that he inflicted heavy losses on Begnion during a border conflict when he was young. War was coming.
Deghensia made it his mission for everyone to think there was a dark god sealed away, so Ashnard wanted to wake this dark god. He had managed to secure Lillia as a captive before the Serenes Massacre took place, and he had knowledge of things like Blood Pacts. He has to have knowledge of the medallion to even want Lillia, so it's safe to assume he knew more than Lehran realized.
But this wouldnt even be Crimea and Daein's first war with each other.They had been at war with each other 220 years prior. Ashnard ruled Daein for 18 years to prepare a new generation (recall the devastating plague) for world conquest .
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Roy's our boy! One man's sunset is another man's dawn.