I see... The reason LEN in the OG game killed the white saint is most likely because of the Demon Stone ( which is most likely a mistranslation of magic stone ) inside her body. Maybe it's one of the catalyst to summon / awaken the demon king, knowing that, the saint told LEN not to speak a single word about it to anyone and also told him to kill her. Why? most likely because it's the only way to stop the summoning / awakening of the demon king. Why him tho? well... the hero might not want to do that and the saint knew it, so she asked LEN instead.