I think the antibiotic example is quite telling. Let's press it a little further. As you correctly point out, killing in self-defense is considered legitimate b

I think the antibiotic example is quite telling. Let's press it a little further. As you correctly point out, killing in self-defense is considered legitimate by most (but certainly not all) people. If someone pulls out a gun and starts shooting at me, most people would agree that it is morally acceptable to pull out a gun and shoot my attacker. But the legitimacy of that self-defense is limited by the principle of proportionality. If instead I pull out an AK-47 and rake back and forth with indiscriminate automatic fire, killing not only my attacker but also 15 innocent bystanders, pretty much everyone would agree that I acted immorally. The death of my attacker would be in self-defense, but I'd still be on the hook for 15 murders. That second example is more akin to taking antibiotics. If we accept that killing microbes is wrong, and accept your argument that using antibiotics is moral because it's legitimate self-defense, we have a problem. Antibiotics kill a lot more than just the invading bacteria. So does radiation and chemotherapy. In fact, they kill a lot of microbes who live in a symbiotic relationship with me and live in my body because they help me break down food or fight infections. The point is, things are a lot more complicated than simply saying that all forms of killing are immoral. At some point we have to accept that there is an ordering of life forms, and killing some of them to save ourselves is acceptable. If we don't, using antibiotics, chemotherapy, radiation, or other similar forms of care crucial to modern medicine is immoral. Once we accept that not all killing is wrong, it becomes an issue of line-drawing.