Well, you would have to say how it is illogical. That it is wrong doesn’t necessarily make it illogical.
Ah, now there I agree with you. I believe (with Lewis) that our universal innate sense of morality points to the existence of God (it doesn’t prove the existence of God, mind you), but that’s different from saying there is (or we have access to) an “absolute answer to every such possible situation.” In fact, I would fear those who would claim such access.
That still leaves the deeper questions of why and how “harm” is a basis for universal or absolute morality… (as opposed to, "I just don’t like it). For example, is it because harm is an obvious and axiomatic negative outcome? Why? Just to people? If I harm an animal is that okay? Is it okay if there is a legitimate reason, say, I need food? You could logically extend the concept of “harm” to the environment. I would agree that it is wrong to “harm” the environment but not to the extent that I wouldn’t live in a house in a suburb (that obviously “harms” the local environment). Is “harm” of humans wrong because humans have innate value? That’s something I would say yes to, but it is not a given that humans have innate value. What makes that true?