They could be rational. A perfect virus that infects every animal on earth and then instantly kills them would decrease the suffering to zero. It's not hard to imagine utility functions where eliminating every human (or animal) is the best choice of action.

Unfortunately, in reality, such attacks are likely to cause huge amounts of suffering which muddies the water.

But if there was a magic button to press that'd instantly eliminate all life on earth, I think it'd only be fair to push it.