Why is this person having such a hard time wrapping his mind around the concept that there really is no such thing as right and wrong?

He posted a (nonsense) video about how Christian values are different (better than) secular values.

One of the comments is:
“An atheist can choose not to murder, but they can’t honestly acknowledge that murder is wrong.”
I responded that ”Well, murder isn’t wrong, but it’s not nice, either, especially for those who loved the person and are left behind. I have no wish to cause someone else heartache and pain.”

I also pointed out that “right” and “wrong” are really man made concepts. The more pain something may cause a person, or others, the more “wrong” we determine it to be. That’s why murdering someone carries a much stiffer penalty than jaywalking.

I think the OP is just flabbergasted and speechless.