Striving for ethical and compassionate behavior toward animals need not be and should not be an all or nothing prospect. For example, a vegetarian who wears leather may very well cause less harm to animals than an omnivore who wears leather.

Restricting one's consumption to vegetables can promote animal welfare despite engaging in other practices that harm animals.

Demoralizing people who strive to promote animal welfare by pointing out their contradictory behavior is less desirable, to my mind, than encouraging people to do as much as they can.

Why not reconsider your own views and encourage such people to promote animal welfare in all the ways they could rather than demoralizing them to give up altogether?

EDIT: spelling, rewrite final sentence, formatting.