Thank you for your response.
Your comparison to the people who claim there are no bad dogs, only bad owners, is a good one. I dislike such claims as they are not only untrue, but they also display a high degree of arrogance. While it is true that a bad owner can cause a generally good dog to be poorly behaved and aggressive, it is also true that a small number of dogs are just plain bad no matter how well they are treated. I've seen a couple examples myself of dogs who were aggressive and downright mean even as puppies, even though I'm pretty sure no one abused them. Of course the opposite is also true: some dogs are gentle and loving even after they have been abused, but we shouldn't presume that this is true of ALL dogs.
You make an interesting point about those who think humanity is so bad that the universe would be better without us existing. This is of course rather silly as without humans (or at least without some type of conscious observer) there is no such thing as value or "better". It also occurs to me that many of the same people are liable to make claims about childhood innocence, so it seems there is some degree of cognitive dissonance involved.
We might call the idea of humanity being so bad that the universe is better without us an endorsement of "original sin", but the Christian faith at least balances this with a God who loves humanity and wishes to redeem it, and a general sense of humanity having great value despite its inherent faults. This seems to me a much healthier view. A valued treasure may lose some of its value by being imperfect or damaged, but it is still worth SOMETHING - and in some cases signs of minor damage and wear make a treasure even more valued than it would be otherwise. Some materials such as concrete even strengthen with age, and muscles only grow stronger with repeated exertion. It is necessary to recognize original sin, but that does not make it the whole story.