As far as I understand it, 'El' or 'Elohim' is syntactically similar to 'God' whilst 'Yahweh' is equivalent to 'Lord' or 'Our Lord'. Different acts and responsibilities in the Tanakh/Old Testament are attributed to different names. El created the world, required the first humans to be fruitful and multiply, then pretty much left them to it. Yahweh handles the more destructive aspects of Genesis, such as the explusion from Eden, the punishment of Cain (incidentally, at the point of Abel's murder, there was no explicit commandment by God not to kill, so how was Cain to know ?) and the Flood. So the name used represents the action/state of mind of the Diety. This is especially emphasised when Abraham and his descendants refer to 'the God of my fathers' when communicating with God. This is in stark contrast to the Israeite elders who were afraid to climb Sinai with Moses in case seeing Yahweh's or heaing His voice would mean their instant death.