"...it hath the primal eldest curse upon't - a brothers murder!"

-Shakespeare

In this, "the primal eldest curse" is reffering to "The Mark of Cain" which was what God cursed Cain with because Cain murdered his brother. In Hamlet a bad murder was committed so they compared it to Cain because fratricide, killing your brother, was seen as unnatural. The biblical allusion was there to show how bad the crime was using something most could compare to.