Only a conscious being would feel pain, so what is consciousness? Does it come down and embed itself into creatures that are born? If so, if they weren't born, would that be necessarily better? What if consciousness, if it didn't have a body to enter into, appeared in a different universe from ours with its own, possibly worse, sufferings? Wouldn't it be better to give the animal a full, relatively pain free life than the possibility that it would suffer greater in some other universe in some other dimension?
Or does consciousness originate on the spot, somehow born along with the creature? In that case it would be better to not be born at all? Or would it be better to have more consciousness in the universe, despite the suffering an individual might experience, rather than otherwise? Is consciousness a good in itself? Would a universe devoid of all conscious beings be the perfect good, lonely, desolate, without meaning. Or one full of them, with shared misery to go around?
All in all, it's seems quite a murky topic to be throwing around judgement on people about.