And that belief is fairly silly. Yes, we are on top of the food chain and yes that makes us the better species in general, but that doesn't give us the right to view us as something greater than any other form of life. They live, they die, we live, we die. Its the same thing.
As well, an unwanted life is much more tragic than a death before sentience. One gets to experience a terrible life. The other one gets the benifit of not living it at all.