This is the first of several biblical fratricides. Brothers in the book of Bereshit seem to live poised on the fault line of an explosive tension connected to covenant, family line and inheritance, with an urgency that can only end in attempted or actual murder.
It is remarkable how often these conflicts take place in the "field".
What is "the field"? The field turns out to be a place of danger, a site unboundaried, exposed and unprotected, far from watchful eyes. The field means the outside world, as opposed to the presumed safety of the home.
Cain mysteriously lures Abel into the field and kills him. Rashi and Sforno - who were surely not only children - have this to offer as commentary: Sforno makes the simple, practical suggestion that being in the field means being away from parental supervision.
Rashi suggests Cain started a quarrel with Abel, any quarrel, to be settled "outside". He understands that there are dark forces at work between brothers - he will later place the epic childhood struggle between Isaac and Ishmael in a field.
The danger is most starkly represented a generation later in the bitterest conflict of them all; Esau is described as the man of the field, while Jacob is indoors in the tent.
The journalist Dith Pran coined the term "the Killing Fields" for the site of the 1970's Cambodian massacres under the Khmer Rouge regime.
Anyone visiting Treblinka cannot fail to be reminded of what can happen in a field at the end of a railway line, far from "civilisation".
Cain makes the fundamental error of assuming that he can do as he wishes once he is in a field.
God's immediate intervention afterwards is a warning that there is a higher witness to human action. What happens - even in a field - will not go unobserved or unpunished.