The Kitáb-i-Aqdas - The Most Holy Book
Index term: penalty for, under Murder
k62.
Should anyone intentionally destroy a house by fire, him also shall ye burn; should anyone deliberately take another's life, him also shall ye put to death. Take ye hold of the precepts of God with all your strength and power, and abandon the ways of the ignorant. Should ye condemn the arsonist and the murderer to life imprisonment, it would be permissible according to the provisions of the Book. He, verily, hath power to ordain whatsoever He pleaseth.
n86.
Should anyone intentionally destroy a house by fire, him also shall ye burn; should anyone deliberately take another's life, him also shall ye put to death.
The law of Bahá'u'lláh prescribes the death penalty for murder and arson, with the alternative of life imprisonment (see note 87).
In His Tablets 'Abdu'l-Bahá explains the difference between revenge and punishment. He affirms that individuals do not have the right to take revenge, that revenge is despised in the eyes of God, and that the motive for punishment is not vengeance, but the imposition of a penalty for the committed offence. In Some Answered Questions, He confirms that it is the right of society to impose punishments on criminals for the purpose of protecting its members and defending its existence.
With regard to this provision, Shoghi Effendi in a letter written on his behalf gives the following explanation:

In the Aqdas Bahá'u'lláh has given death as the penalty for murder. However, He has permitted life imprisonment as an alternative. Both practices would be in accordance with His Laws. Some of us may not be able to grasp the wisdom of this when it disagrees with our own limited vision; but we must accept it, knowing His Wisdom, His Mercy and His Justice are perfect and for the salvation of the entire world. If a man were falsely condemned to die, can we not believe Almighty God would compensate him a thousandfold, in the next world, for this human injustice? You cannot give up a salutary law just because on rare occasions the innocent may be punished.

The details of the Bahá'í law of punishment for murder and arson, a law designed for a future state of society, were not specified by Bahá'u'lláh. The various details of the law, such as degrees of offence, whether extenuating circumstances are to be taken into account, and which of the two prescribed punishments is to be the norm are left to the Universal House of Justice to decide in light of prevailing conditions when the law is to be in operation. The manner in which the punishment is to be carried out is also left to the Universal House of Justice to decide.
In relation to arson, this depends on what "house" is burned. There is obviously a tremendous difference in the degree of offence between the person who burns down an empty warehouse and one who sets fire to a school full of children.
n87.
Should ye condemn the arsonist and the murderer to life imprisonment, it would be permissible according to the provisions of the Book.
Shoghi Effendi, in response to a question about this verse of the Aqdas, affirmed that while capital punishment is permitted, an alternative, "life imprisonment", has been provided "whereby the rigours of such a condemnation can be seriously mitigated". He states that "Bahá'u'lláh has given us a choice and has, therefore, left us free to use our own discretion within certain limitations imposed by His law". In the absence of specific guidance concerning the application of this aspect of Bahá'í law, it remains for the Universal House of Justice to legislate on the matter in the future.