You’re right about one thing: “Killing is wrong” is broken. I say “the principle is never the principle.” People invoke high-sounding principles to claim the moral high ground but the “principles” quickly get entangled in contradictions. Killing is wrong? War, Capital Punishment? Yep. Assisted suicide? Oh, that’s not killing….”
The problem with your whole analysis is it’s based on your subjective emotions. So what happens when somebody says “screw you and your feelings. This is what I think is right?”
We had a street person that used to wander in to our residence in college, and he’d wander to the music room and play this one maudlin Peter, Paul and Mary song over, and over, and OVER, and OVER again, until he’d finally be invited to leave. During one of these evictions he yelled “Jack Kerouac is a friend of mine, and I’m gonna tell him about you, and he’ll write in his next book that you guys are not cool!” Yes, he really said “not cool.” No, I am nowhere near creative enough to make that up.
Our street person was amusing for coming up with the most toothless threat ever, but subjective morality is hardly better. So when homophobes say “We’re going to outlaw gay marriage, because we believe that’s right, and allowing it to persist makes us complicit in a wrong, and we don’t care what you think,” what are you going to do, tell Jack Kerouac on them? Even more, what gives you the right to try to legislate against their position, based solely on your subjective appraisal?
As for the alleged complexity of “killing is wrong, but…” just look at say, a leaf. Sunlight hits the leaf and triggers a cascade of complex reactions that would take a lengthy book to describe in detail, but the leaf sorts it all out. Similarly, it takes a lot of words to describe the morality of killing, but in practice we sort it out fairly smoothly. C.S. Lewis once wrote that we can expect morality to be at least as complicated as physics.
One of the biggest canards around is that modern morality is flexible and situational, whereas traditional morality was rigid and inflexible. The truth is exactly the opposite. Traditional morality always took note of situations, so much so that it created specialized terms for specific combinations of actions and circumstance. The morally neutral term for taking human life is “homicide.” War, self-defense, manslaughter, murder and capital punishment are all terms for homicide plus a situation. It’s the modern moralist who makes rigid blanket statements like “All killing is wrong.” (Did you ever wonder why it says “Thou shalt not kill” but “David slew Goliath? Welcome to the pons asinorum.) The morally neutral term for sex is, well, “sex.” Marital intercourse, adultery, fornication, masturbation, and homosexuality are terms for sex plus a situation. Ask a traditional moralist if poverty is wrong and he’ll ask “why is the person poor?” It’s the modern moralist who announces dogmatically that poverty is wrong.