German animal-lovers are divided on the issue of "Cute Knut", an orphan polar bear that has (so far) been raised by human beings at the Berlin Zoo. Some argue that the bear should have been euthanized, for "[i]f a polar bear mother rejected the baby, then ... the zoo must follow the instincts of nature." And like-minded others argue that "[t]he animal will be fixated on his keeper and not be a 'real' polar bear," and that being raised by humans is "not appropriate to the species."
As Thom Yorke said, we hope your rules and wisdom choke you. The very idea that a being would be better of dead simply because it's not living a "natural" life -- well, I guess I just can't make sense of it. Let me try to explain why. Here a situation in which it seems like you might be better of dead if you couldn't live a natural life: you're a fish, and you're not allowed to live in water, but when you're outside of water you are in constant pain and suffering from lack of oxygen, and (this may go without saying) you don't have any important Williamsonian ground projects that involve being out of water. If that's you, then you'd be better off dead.
But that is because the unnatural life that you're forced to live is a life of pain and suffering (with no compensation when it comes to the things you really care about). It's not the mere fact that your life is unnatural that makes it a life not worth living. What ultimately explains why your life is not worth living, in that case, is not the fact that your life is unnatural. Rather, it's the pain and suffering.
In the case of a fish, intuitively, there may be nothing more to the good life than pleasure and pain -- any fish life that isn't a life of suffering is a fish life worth living, for a fish. How might other animals -- including humans -- be different? The existentialists (and American sympathizers like Frankfurt, Nagel, and Williams) have an answer to this question: (roughly) human beings have values, i.e. things they love and care about, i.e. projects that give meaning to their lives. On a less grand scale, many animals (including humans) have interests, desires, concerns, preferences. To the extent that you have these sorts of wants (whether they rise to the level of existentially significant "ground projects" or not), your life is worsened by not getting what you want, in the same way that its worsened by pain and suffering. So the question we have to ask about Cute Knut, it seems to me, has nothing to do with the biological nature of bears, and everything to do with what he wants, and whether he's getting it.
Can we attribute to Knut a ground project of wanting to be a "real" bear, according to zoological standards? Of course we can't -- what Knut cares about is getting food, and playing with a toilet brush, or whatever. He could care less about living up to his biological nature, if he even things about such things, which he doesn't -- and that's the real point. What's "natural" can't matter to a being that doesn't (and probably can't) form thoughts about what's "natural." And if it can't matter to Knut, then it can't be the difference maker for whether his is a life worth living.