His Bond 1 line says: "Originally I'd be a Ruler or a Berserker. The reason why I'm an Assassin this time is because there was a guy who gave up their post due to previous circumstances. There's no one else who can do it, so I couldn't help but inherit it. …I'm serious, y'know?".
He lost his memories AFTER this, probably because the pain from having to kill her broke him.
One of his bond lines states that he would've become a counter guardian anyway, but this event is what changed him into what we know of him now.