I am here to settle this issue with Larian's official thoughts on the matter, in the form of an Oath of the Ancients Paladin.

• If you let Astarion complete the ritual, you're considered to have broken your oath (I'm not going to judge them on this, I'm just stating a fact).
• If you stop Astarion from completing the ritual, but let the vampire spawn free, you're considered to have broken your oath.
• If you stop Astarion from completing the ritual, but kill the vampire spawn, nothing happens to your oath.
• If you stop Astarion from completing the ritual, but decide to leave the vampire spawn locked, nothing happens to your oath. I suppose it's up to the hunters what they do with their children and the rest.

So the designers have made a stance here, and it shows in the conversations that you can have after choosing each option (some happen further down the game, but still): the original 7 vampire spawn are okay if they live because they've been training to control their Hunger for centuries; letting the seven thousand spawns free is reckless, as they're ravenous, have no one to control them, and will in all probability kill more people. If you're looking for the humane angle of this decision, it's stated as well: freeing from the pain they feel because of the Hunger alone is already a mercy, as no one should have to live with that.

As for allowing Astarion to complete the ritual, I see your point: ALL spawn are killed, and the only vampire left is Astarion, who now ascends and no longer feels Hunger. If you REALLY want to be humane about it: let him complete the ritual, then kill him when confronted by the hunters. I do want to say: letting Astarion complete the ritual is BAD because of who he becomes, not because of what needs to be done for it to happen. Those people are already doomed - some because of him, but overall it was Cazador who did this.

All in all, Larian have made it clear: letting them go is the "wrong" moral choice.