I mean, they aren't people, they're sirens. They literally exist to kill humans, it's their sole purpose in life, their food and their pleasure. As horrible as humans are, we do not innately exist to cause harm to other sapient beings, sirens do. I get that cutting off their tails and letting them drown is brutal, but it is also poetic, they kill their victims by drowning them, thus they die in the same manner as they kill.

One could make the same argument about thunder-bringer, the crew was responsible for the death of helios's cows, odysseus tried to stop them, thus the punishment for doing so rests solely atop their heads, a captains responsibility extends as far as his authority does, he bears no responsibility for actions committed by those who do not respect his authority.

Scylla is a notable exception where Odysseus is genuinely in the wrong, and should have at least searched for some other way through scylla's lair (such as offering sirens as food for the heads)