“Murder” is an illegal killing. I don’t oppose murder; I oppose immoral killing. That’s different.
If you simply claimed that you’re against pointless killing I wouldn’t consider that arbitrary, since I share your strong intuition that causing meaningless suffering is deeply wrong. That is, in fact, precisely why I find it confusing that you would violate this intuition.
An arbitrary moral distinction would be like claiming that you are against ending innocent lives, unless they’re a different race, gender, species, nationality, or color than you, given that none of these factors have any moral relevance.
What is the moral significance of a creature’s nationality or species? Moral philosophers consider this question fairly settled, so let me know if you have some novel insights.