Well, if we lived on the Klingon homeworld, 1) would be more just, in fact, it would probably be considered honorable.
Since, we do not, however, then I must respond with: "insufficient information."
1) Assuming that the son of the murderer had absolutely NOTHING to do with the murder of the victim, and he was not responsible in ANY way for any obstruction of justice that may have taken place in order to protect his father, then his killing by the victim's family is not a mere killing - it is a murder. An unjust murder.
2) A man is killed. Ok... men are killed every day. Who is this man? What did he do in the last few months of his life? Who killed him? Why? Without this information, I'd consider the life of this mystery man to be just as valuable as that of the son murdered out of vengeance.
-Warik