Because theres no "harm" being done. Its already dead so it doesn't care, nobody is around together traumatised so they dont care.

Its obviously wrong but without Daddy God telling you its naughty its not so easy to explain why. I kinda want to see the Bible passage that says "dont fuck an animal corpse ye faithfulle"

This is a common "gotcha" argument that floats around the Internet. Essentially, it posits that under the teachings of a given religion (typically Christianity), there is a source of morality which is absolute. That might be the religion's holy text, deity, or the religious authorities of that religion. In either case, there is one authoritative source which dictates what is morally correct and not correct for the entire universe from now until eternity.

So, the argument goes, if one is to reject the teachings of this or all religions, as irreligious people do, it necessarily means accepting that morality is inherently relative and that there is no absolute standard for mortality that is universally applicable. Therefore, as the argument goes, since one would have to accept that morality is relative, it can be framed relative to anything or nothing, and therefore there is no act which can be immoral relative to any reference frames in a context without religion. And therefore, nothing can be said to be immoral because whether it is moral is relative.

That's the end of the argument.

To its credit, there isn't anything wrong with this argument. But I do believe the argument posits that conclusion to be far worse than it really is. Suppose I am an irreligious person. Why is fornication with roadkill immoral? Well, because I think it is. It makes me feel bad and the reward gained isn't worth the risk (the embarrassment of being seen in the act or catching some disease from it). Therefore I don't do it. Is it possible that some person could think that it is moral? Yes, absolutely. But that doesn't matter, because even if relative to one person's moral compass an act is moral, doesn't mean that people in general can't just collectively reject that perspective and condemn the act as a group. In fact, human societies imposing their views on what is and is not moral relative to their own experience describes pretty much the entirety of human history.

Edit: To sum up, my counter argument is that yes, all morality is relative. I don't see how this is a bad thing. Humans have the ability to reason and reject moral viewpoints which they collectively find repulsive. They do not have to accept it just because someone else thinks that way.

corpses are dead and cannot receive a moral injury.

(setting aside the potential to cause a plague outbreak, which is a morality issue IMO)

Such as if you paid a friend to run it over.

That's still you committing a roadkill, just second hand.