GEN506 Repentance for murder is inevitably incomplete, because reconciling with one’s victim is impossible.  Furthermore, murder is often a sin that extends even beyond the victim’s life span.  Thus, after Cain murders Abel, God says to him [this verse]. The word used for blood, d’mei, is plural, so the verse literally reads, “the bloods of your brother.”  From this plural usage, the Talmud deduces that what cries out is not only Abel’s blood, but also that of all his future, never-to-be-born descendants (Mishnah Sanhedrin 4:5).  This is an additional reason why it is impossible to fully atone for murder.  TELVOL I:184-5