Sin is merely God's opinion. There are millions and millions of people who would disagree with Yahweh's opinion. How can one person's opinion be more right than another persons?
Sins are sins because they harm other people, they aren't just arbitrarily called evil because of some hidden agenda that God has. Saying that people disagree with God only proves that God is right because, as a whole, people overwhelming choose to do evil.
I'm sure you would agree the Holocaust was a terrible thing, right? Obviously Hitler didn't kill ~21 million people on his own, a large majority of Germany population supported him. And what about the USSR? That government was responsible for an estimated ~62 million people murdered. Again, a handful of people can't kill 62 million, there were people who supported their government during this democide. The People's Republic of China is the bloodiest government to date, with nearly 80 million people murdered. And none of this even includes wars, or abortions which kills ~40 million people each year. Totaling all the murders in the last century would probably exceed one billion. You'd have to be pretty ignorant about history to claim that people are capable of deciding for themselves what is right and what is wrong.