I think it's always been the case that there's been 'demonic' or other forces that cause us to behave badly or to make the wrong decisions so that, accordingly to ideas about reincarnation, we need a theology or philosophy that says 'behave badly and you will come back as a slug' or 'dont do the right thing or you'll go to hell'. Perhaps 'these days' we have less people subscribing to religious ideas that would moderate their behave or perhaps we just have more people or we have the media to inform us and make us believe their are 'demonic' or equivalent forces.
Bottom line is we seem to need to think there's a reason to behave well for some after this life reward, rather than having an intrinsic motivation to be a good person in this ONE life.
Sumerians were just philosophising like human have over the centuries - that's the rub right - no one can be sure.