Generally speaking, I think we decided long ago that certain things weren't acceptable in a civilized society. We made up gods and made up the idea that these rules come from him (usually a him). That sorta gave those beliefs, his beliefs, a little more punch to them. Doing something to tick off the gods is a lot more scary than doing something wrong in the eyes of your fellow man. Why, you might even call it a sin
His morals come from whatever we assign to him.
You'd be surprised at how many cultures have/had similar fables to the ones some believe in today. One of the main ones is that god, being mad at men and his sinful ways, sent a teacher to earth in order to straighten us out, or save us if you're a christian.
It's an amazing coincidence how similar some of these are, especially when there's almost no way all of them borrowed from each other.
I don't personally see anything wrong with myths if people take good lessons from them and live good, morals live because of such things. The problem is always when some think their god is the one true god/way and that all heretics can either convert or die.