Here's the
quote from Baha'u'llah. Then one from the Guardian of the Baha'i Faith. And then the Baha'i Universal House of Justice
"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."
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.
"...merely the fundamental principles of the punishments for murder and arson are given in the Kitab-i-Aqdas. Willful murder is to be punished either by capital punishment or life imprisonment. Such matters as degrees of offense and whether any 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 in operation. Arson, as you yourself can see from the newspapers, is becoming an increasingly frequent offense-scarcely a day passes without some building being burned or blown up, often causing agonizing death to innocent people. Bahá'u'lláh prescribes that a person who burns a house intentionally is to be burned or imprisoned for life, bur again, the application of these punishments, the method of carrying them out and the fixing of degrees of offense are left to the Universal House of Justice. Obviously there is a tremendous difference in the degree of the offense of a person who burns down an empty warehouse from that of one who sets fire to a school full of children."