How to devise moral principles

Why choose 'death of a conscious, pain-feeling life-form' as your arbitrary distinction?

Do you think this defintion holds up for the purposes of defining meat-eating as harmful?

My quibble is with the link between harm and morality. I see no reason to base morality on causing harm. I chose to lead into this by pointing out that you have already qualified 'harm' as 'harm to pain-feeling, conscious animals'. The more qualifications one introduces, the closer one might come to your opinions, but these qualifications really need some sort of justification. Since the whole argument rests on this proposition, I think that discussing it further is worthwhile.

I think that the further qualifications come not through any set of fundamental principles that lead to them, but in an attempt to rationalise moral beliefs that are already held. This sort of 'post hoc justification' is common (as far as I know) in philosophy and science, but is really not good enough.

I suggest that we start with looking at why we do not cause harm to others and defining very clearly a moral principle there before we start trying to apply it to practical cases.
If we always think of the practical cases as we're devising a principle we might bias our ideas of what is logical by trying to make the principle's conclusions fit our preconceived notions rather than finding a sound principle and applying it in order to decide what our notions should be.