This is a shallow understanding of atheistic morality. For context, I’m Christian and was sent to religious schooling from k-12 and had to take an ethics course in 11th grade. Yes we talked about Christian morality, but we also talked about historical ethical thought and philosophy. Naturally we focused on Greek thought bc it’s the west; we love Greeks. Some philosophers prioritized happiness and pleasure, so if fucking roadkill makes you happy, then it’s good. Others prioritized public benefit, so if fucking roadkill actively distresses other people of the public, even if it makes you happy, then it’s not good. Also, most people, regardless of religion, treat other living things with dignity simply because it feels right to do so. Most people have empathy and can think “I wouldn’t like that done to me” or “I would feel disrespected if that happened to me.” You don’t need God to feel bad about fucking roadkill. You just need normal human empathy and compassion

Even putting aside all that though, it’s pretty common in Christian thought to believe that God gave people a thing called a conscience that for most people, whether they are Christian or not, helps guide them to do good things and not do bad things. So if you believe in Hod (it was meant to be God but autocorrect had a funnier idea) then there’s a really obvious answer for why atheists wouldn’t fuck roadkill. What’s not obvious is why this person was thinking about fucking roadkill this much to begin with