After all, is good and bad are just a matter of opinion, then we are justified to come to AC41363484's house and rob all of his money if we want to. In my opinion, it'd be cool if I had all of that money. And if AC doesn't agree, well, that's just the way he sees it. My opinion is equally as valid, anyways. None of them is objectively better than the other.

If someone is a sadist, then may somehow decide that they want to torture AC41363484's mom. And AC41363484 can't be angry about it, because the sadist didn't do anything wrong. AC can't say that that's a bad thing that the sadist did to his mom. Why should his opinion count more than the sadist's?

Surely, no one sane would believe in what AC is saying. No one would hold AC's opinions about the world. I don't think it's good to believe in such things!

So, in my opinion, good and bad things exist objectively. Seems to make more sense to me.