Should anyone unintentionally take another's life, it is incumbent upon him to render to the family of the deceased an indemnity of one hundred mithqals of gold. Observe ye that which hath been enjoined upon you in this Tablet, and be not of those who overstep its limits.

Bahá’u’lláh, Kitab-i-Aqdas, #188

As to the question regarding the soul of a murderer, and what his punishment would be, the answer given was that the murderer must expiate his crime: that is, if they put the murderer to death, his death is his atonement for his crime, and following the death, God in His justice will impose no second penalty upon him, for divine justice would not allow this.

‘Abdu’l-Bahá, Selections from the Writings of ‘Abdu’l-Bahá, p. 179

If the community and the inheritors of the murdered one were to forgive and return good for evil, the cruel would be continually ill-treating others, and assassinations would continually occur. Vicious people, like wolves, would destroy the sheep of God. The community has no ill-will and rancour in the infliction of punishment, and it does not desire to appease the anger of the heart; its purpose is by punishment to protect others so that no atrocious actions may be committed.

‘Abdu’l-Bahá, Some Answered Questions, p. 269

All men are in God's hands, and even if they do get killed we know there is another life beyond this that can hold great hope and happiness for the soul.

Shoghi Effendi, Lights of Guidance, p. 237

You enquire whether you should take action to have your parents charged with murder, following the death of your brother. You should ascertain from a competent lawyer what are your legal obligations in this regard, and follow such requirements. If there is no legal obligation, it is left to your discretion to decide on this matter, in light of the circumstances However, you might well ask yourself, in the course of this decision-making, what beneficial result is to be gained from such an action, more especially if the action occurred some years ago and if legally-acceptable proof is difficult to establish; you should also weigh carefully the effect this might have on yourself, in the process of re-opening the subject, testifying about it in court, and doubtless incurring the antagonism of your parents.

The Universal House of Justice, 1985 Dec 02, Child Abuse, Psychology and Knowledge of Self

I ASKED ‘Abdu’l-Bahá concerning the state of all those young men souls who have fallen so suddenly and tragically during our world war. Could they affect our present living conditions in any way?

"His reply ran as follows: "God treats these people with His mercy, not with His justice, since God is against war. But as many did not will the war, but were obliged to go to the battle field by force of circumstances, therefore God has mercy for they suffered much and they lost their lives. These deserve the forgiveness of God. As they suffered in the world and were afflicted by great calamities and their blood was shed and in reality they were treated unjustly and thus died unwillingly, therefore God will have mercy and forgive their shortcomings and will reward them. He will compensate them for loss. Is it just to be so afflicted and killed and suffer and have no reward? This is contrary to the Kingdom of God. We supplicate God that these murdered ones will become and stay alive in His Kingdom and be submerged in the sea of His mercy and be happy."

Star of the West, Volume 10, Issue 18, pg. 336