In the Bible God has always intervened in human affairs.Backing their wars against other nations inciting genocide,even sending Satan down to the Earth so he could stir up more strife among humans .
Why didn`t he send Satan to Ur-anus ,where he couldn`t interfere in human affairs.?
As far as God`s dealings with Cain he just isn`t consistent with his own laws and changes where it suits him.
An eye for an eye and do not murder etc.etc.