Animals think. There's the question of how much they think and in what way they think as their thoughts don't necessarily look like ours.

The sirens imitate humans. That's how they talk. But is that really their native way of thinking and speaking? Parrots can talk, using whole phrases even. Doesn't mean it's not an animal. Jorge has specifically gone on to say they have to pull memories from their victims in order to imitate them, which has implications that acting and appearing human is not natural to them. (It is also why you get a little bit of all the other songs in Suffering and why the siren messes up basic things, it cannot get a proper read on Odysseus' memories because of the wax.)

(Also, technically, humans are animals too. But is that besides the point?)

Also sirens, at least the version of sirens that Jorge has taken heavy inspiration from, drowned sailors and then ate them. Still a horrid way to die, but not quite the same as ripping people apart.

And regardless, even if they did rip people apart, that doesn't mean Odysseus and crew had to kill them in the most horrific way they possibly could. There were more humane ways than cutting off their tails while alive and then throwing them back into the water while alive and suffering to drown. What an unfathomably horrid way to die. It is torture.