Quote from FusionYou are saying that we should kill innocent aliens because they *could* kill people, which is the same as saying we should kill innocent black people because they *could* rape white women or smoke crack cocaine. Neither of these things are reasonable because you are deciding you should kill them based on a characteristic they don't have, which is almost like killing them for no reason.
They *could* kill people? Well, they have just destroyed a half of the planet. So there is a high probability for them to keep destroying. If we would decide to take the risk we could be completely wiped out.