> There is a good argument to be made that in the West we should eat less meat, but that doesn't mean removing it completely: it is harder to make a balanced vegan diet, so for most people that would mean a less healthy outcome that just eating a moderate amount of meat.

You're making something of a jump there - not eating meat equates to a vegetarian diet, not a vegan diet. One can argue about the merits of a vegan diet over a vegetarian one, but a vegetarian, non-vegan diet is already a big step up over a meat-heavy one, and a vegetarian diet including eggs and dairy doesn't really have any challenges in being balanced/healthy compared to a moderate-meat diet.

> Why should explain why something is NOT bad?

Because they're the one who made the claim without evidence or argument.

> It's up to you to convince him (or others) that it is bad

Yes, that's what I did.

> the reasoning "according to MY ethics it is bad" is not a good argument.

That wasn't my argument.