@Night Ripper,
If even one life is not worth living, that comes at the cost of everyone else's decision to perpetuate life for the worth of those that are.
Similarly, though I do not believe in hell, some do. The quality of an action being described as unethical or evil depends on the understanding of the one who is acting on it. If you believe even one person born will go to hell, then perpetuating life towards that inevitability is evil. Again, I don't think the hell model is real, its imaginary, but it demonstrates that people are valuing the good of some lives against the bad of others.
Hell aside, there ARE lives that have been just completely miserable and of no note whatsoever. Human life is not the only life either. There are billions upon billions of pointless existences, punctuated by fear, agony, and a gruesome death. There are many examples of unbearable and fruitless tragedy in human lives as well.
And biology does prevent suicide in most cases. Our enjoyment, desires, and distractions from the bad come from a system adapted specifically for self-preservation and progeneration. In some cases the immediate stimuli of the bad outweigh this system and suicide occurs.