No believer should kill another believer intentionally. In case of unintentional man slaughter (such as car accident), the killer must set free a believing slave and pay blood money to the heirs unless the latter do not demand it to please God. If the believing victim lived among the enemies (and was killed say in a military campaign by mistake), the expiation will be freeing a believing slave. If the believing victim lived among the non-believers with whom you have a peace treaty, the expiation will be freeing a Muslim slave and the payment of the blood money to his heirs. If one (for monetary reasons) can not free a slave, then he has to fast two consecutive months and ask for forgiveness from God, the Most Knowledgeable, the Most Wise.