Prime Minister of Persia issued “an explicit order from his sovereign” to the governor of Ádhirbáyján… to execute the Báb” in Tabriz

The siege of Zanján was still in progress when he, dispensing with an explicit order from his sovereign, and acting independently of his counsellors and fellow-ministers, dispatched his order to Prince Hamzih Mírzá, the Hishmatu’d-Dawlih, the governor of Ádhirbáyján, instructing him to execute the Báb. Fearing lest the infliction of such condign punishment in the capital of the realm would set in motion forces he might be powerless to control, he ordered that his Captive be taken to Tabríz, and there be done to death. 

- Shoghi Effendi  (Chapter 4, 'God Passes By')

Prime Minister of Persia felt a “more drastic and still more exemplary punishment… must now be administered” to the Báb: - execution of the Báb

While the convulsions of Mázindarán and Nayríz were pursuing their bloody course the Grand Vizir of Násiri’d-Dín Sháh, anxiously pondering the significance of these dire happenings, and apprehensive of their repercussions on his countrymen, his government and his sovereign, was feverishly revolving in his mind that fateful decision which was not only destined to leave its indelible imprint on the fortunes of his country, but was to be fraught with such incalculable consequences for the destinies of the whole of mankind. The repressive measures taken against the followers of the Báb, he was by now fully convinced, had but served to inflame their zeal, steel their resolution and confirm their loyalty to their persecuted Faith. The Báb’s isolation and captivity had produced the opposite effect to that which the Amír-Nizám had confidently anticipated. Gravely perturbed, he bitterly condemned the disastrous leniency of his predecessor, Hájí Mírzá Áqásí, which had brought matters to such a pass. A more drastic and still more exemplary punishment, he felt, must now be administered to what he regarded as an abomination of heresy which was polluting the civil and ecclesiastical institutions of the realm. Nothing short, he believed, of the extinction of the life of Him Who was the fountain-head of so odious a doctrine and the driving force behind so dynamic a movement could stem the tide that had wrought such havoc throughout the land.

- Shoghi Effendi  (Chapter 4, 'God Passes By')

The Báb had “foreshadowed His own approaching death”: - “the sixth Naw-Rúz after the declaration of His mission would be the last He was destined to celebrate on earth.”

A fast ebbing life, so crowded with the accumulated anxieties, disappointments, treacheries and sorrows of a tragic ministry, now moved swiftly towards its climax. The most turbulent period of the Heroic Age of the new Dispensation was rapidly attaining its culmination. The cup of bitter woes which the Herald of that Dispensation had tasted was now full to overflowing. Indeed, He Himself had already foreshadowed His own approaching death. In the Kitáb-i-Panj-Sha‘n, one of His last works, He had alluded to the fact that the sixth Naw-Rúz after the declaration of His mission would be the last He was destined to celebrate on earth. In His interpretation of the letter Há, He had voiced His craving for martyrdom, while in the Qayyúmu’l-Asmá’ He had actually prophesied the inevitability of such a consummation of His glorious career. Forty days before His final departure from Chihríq He had even collected all the documents in His possession, and placed them, together with His pen-case, His seals and His rings, in the hands of Mullá Báqir, a Letter of the Living, whom He instructed to entrust them to Mullá ‘Abdu’l-Karím-i-Qazvíní, surnamed Mírzá Aḥmad, who was to deliver them to Bahá’u’lláh in Tihrán. 

- Shoghi Effendi  (Chapter 4, God Passes By)