My question is, somebody wanted to kill a person, when he fired the gun he missed and killed a by passer. A third unknown person then killed the murderer. Who is suppose to bay the blood-money of the by passer? If it is his family can anybody else contribute to pay it?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

The blood-money when accidentally killing a person has to be paid by the relatives of the killer from his father's side. If some organizations or people want to pay the blood money [on his behalf] or help in its payment, then there is no harm in this. 

Allaah knows best.