Huh? The issue isn’t whether we need morals but how we can construct those morals. I am arguing that the scientific discovery of what is good for physical or mental health is sufficient for morality and the so called is/ought gap is empty rhetoric.
So your question is how does that give you a conclusion that worsening the health of someone else is bad?
The answer is the application of logic to the fact of human interdependence. The worsening health of someone else would mean the loss of efficiency in a potential ally in maintaining your own health.
If we want a real moral challenge then it would be when the needs of two people come into conflict. Suppose you need a new kidney then how would it be wrong to go take someone else’s kidney? If your own physical health is the only standard then I haven’t worked our how that would be bad. Remember, I constructed my moral framework from the basis of psychological health not physical health.