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.

It hath been decreed by God that, should any one of His servants intend to travel, he must fix for his wife a time when he will return home. If he return by the promised time, he will have obeyed the bidding of his Lord and shall be numbered by the Pen of His behest among the righteous; otherwise, if there be good reason for delay, he must inform his wife and make the utmost endeavour to return to her. Should neither of these eventualities occur, it behoveth her to wait for a period of nine months, after which there is no impediment to her taking another husband; but should she wait longer, God, verily, loveth those women and men who show forth patience. Obey ye My commandments, and follow not the ungodly, they who have been reckoned as sinners in God’s Holy Tablet. If, during the period of her waiting, word should reach her from her husband, she should choose the course that is praiseworthy. He, of a truth, desireth that His servants and His handmaids should be at peace with one another; take heed lest ye do aught that may provoke intransigence amongst you. Thus hath the decree been fixed and the promise come to pass. If, however, news should reach her of her husband’s death or murder, and be confirmed by general report, or by the testimony of two just witnesses, it behoveth her to remain single; then, upon completion of the fixed number of months, she is free to adopt the course of her choosing. Such is the bidding of Him Who is mighty and powerful in His command.

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.

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.

it behoveth her to wait for a period of nine months, after which there is no impediment to her taking another husband
In the event of the husband’s failure, either to return at the end of the specified period of time or to notify his wife of a delay, the wife must wait nine months, after which she is free to remarry, though it is preferable for her to wait longer (see note 147 for the Bahá’í­ calendar).
Bahá’u’lláh states that, in such circumstances, should news reach the wife of "her husband’s death or murder", she must also wait nine months, prior to remarrying (Q&A 27). ‘Abdu’l-Bahá, in a Tablet, has further clarified that the nine months' waiting period following news of the husband’s death applies only if the husband had been away at the time of his death, and not if he dies while at home.

Question: Concerning the sacred verse: "If, however, news should reach her of her husband’s death", etc.
Answer: With reference to waiting a "fixed number of months" a period of nine months is intended.