Sanctity of life

I remember reading in a novel once the statement, "all life is infinitely precious." I remember thinking that sounded a little extreme. Are we talking bacteria here, or only those things that we can see as live? Or, do we move even up higher on the chain? I had an associate tell me she didn't eat anything that had a face. So, if it has eyes and a nose, its life is important; no nose, no sanctity?
But let's limit the discussion to humans, and say, "All human life is precious." I simply can't agree with that. But first, some background.
The idea that life is sacred has two sources, I think. The first is religion. We have no right, righteous people argue, to take over the powers of God. Note though, that that's not an argument for the sanctity of life. After all, God kills people left and right. The religious argument is that we're not God; ergo we can't decide.
The second argument is more along the lines of "If we say that one person's life isn't as important as another's, where do we stop?" Kind of the camel-with-its-nose-in-the-tent argument. The argument assumes that if we kill really bad guys, then we will progress to not so bad guys, to merely useless guys, to old guys, to helpless guys, until there's an orgy of destruction that spreads through the land like a plague ("guy" here is used as a trans-gender term). And yet, we make value decisions of the same kind, if not the same magnitude, every day. In fact, part of the human makeup is the ability to make clear-cut decisions on items that are really on a spectrum. There is no reason at all to assume that once we decide life is not inherently precious we'll kill our neighbors for their lawnmowers.
I believe that life is mysterious, wonderful, surpasses understanding, something that we can't create, etc. etc. etc. I do not believe, though, that life is "infinitely precious," not even human life. Some people are just better off dead.
But I'm not about to bump off people I think are a menace. For one thing, they kill you for doing that sort of thing.