To propose an answer to your first question about why so many different cultures have come to the same conclusion about there being a divine entity or entities. When primitive man looked at the world he lived in he had no idea why things were the way they were. There was no knowlege of biology and how living things work. He didn't know how birds could fly or fish could breath under water. The most logical rational conclusion was that their must have been some sort of a all powerful creator or presence to make them do this things.
And secondly to your question why there seems to be a set of universal rules such as don't kill one another. Humans are societal creatures. There is survival in numbers. You may hate Bob and want to kill him, but he usually bags a deer every week so hes usefull to your survival. why kill him if it means you will have a decreased chance of survival? Plus Bob could be good to help fight off any sort of preditor that may wonder into camp. People who stayed together survived and people who killed everyone till they were alone died. It could be argured that this "don't kill other people" mechanism gradually became genetically wired into our DNA as communities became more successfull over time. People eventually spread to all corners of the globe and that could possibly be why people tend to not want to kill everyone.