At least Reincarnated as a Sword has a plot point for why the evil shit is a thing. I don't think the manga has gotten far enough for the full explanation, but it's been at least hinted at.
S-kyuu Guild wo Tsuihousareta kedo, Jitsu wa Ore dake Dragon no Kotoba ga Wakaru no de, Kidzuita Toki ni wa Ryuu Kishi no Chouten wo Kiwametemashita. It's on mangadex but it doesn't update super regularly.
It feels like with these series it's inevitable.
Most series do it nearly INSTANTLY. A few don't do it for quite. There's another series I was spoiled a bit on where I know it happens eventually but it hasn't happened yet. It's
There's a fair argument that keeping one alive to interrogate later is the wisest move, regardless of morality on killing them or not.
But, as the MC said, keeping one of them alive. Keeping both would have been preferable since one might have information the other does not.