Morality is subjective
Advocates of subjective morality wouldn’t tolerate such subjectivity if they were the victims. If someone kills a human, or an animal, and truly believes there’s nothing wrong with this, subjective morality states that this wouldn’t be unethical. Morality must be based on facts and reason, it can’t be completely arbitrary, or else anyone can justify any atrocity by stating that their morality is subjective. We must have at least some objective measurement of what is and isn’t ethical. Agreeing that killing beings for pleasure or convenience isn’t ethical is a good place to start to prevent violence towards humans and animals. Veganism follows from this.
Even if we believe morality is subjective, it’s likely that most people would agree that animals have some moral value and shouldn’t be harmed for no reason. So by this subjective morality, we can agree that veganism is right because harming animals unnecessarily (we don’t need to eat them to live healthily) is wrong.