What is so sacred about life?

I think the idea is that every living thing serves a certain purpose and belongs somewhere, and should not be destroyed without reasonable cause. Personally, I hate spiders and centipedes. But spiders and centipedes like to eat other nasty disease-spreading bugs, so when I see them out in my garden, I leave them alone. That's their territory. If I find them in my house, though, they're dead. They serve no useful purpose there, and can do more harm than good when they're out of their element.