The Angels smite the Disbelievers upon capturing Their Souls
Allah says, if you witnessed the angels capturing the souls of the disbelievers, you would witness a tremendous, terrible, momentous and awful matter,
يَضْرِبُونَ وُجُوهَهُمْ وَأَدْبَـرَهُمْ
(they smite their faces and their backs), saying to them,
وَذُوقُواْ عَذَابَ الْحَرِيقِ
("Taste the punishment of the blazing Fire.")
Ibn Jurayj said that Mujahid said that,
وَأَدْبَـرَهُمْ
(and their backs), refers to their back sides, as happened on the day of Badr. Ibn Jurayj also reported from Ibn `Abbas, "When the idolators faced the Muslims in Badr, the Muslims smote their faces with swords. When they gave flight, the angels smote their rear ends."
Addressing the disbelievers in the second verse (51), it was said that the punishment of this world and that of the Hereafter was nothing but what they had earned with their own hands. Since things done usually take their shape through the work of hands, therefore, mention was made of hands. The sense is that the punishment coming to them was a direct outcome of their own deeds. And as for Allah Ta` ala, He was not the kind of authority who would bring injustice upon His servants and go about subjecting someone to punishment just for no reason.
Tazkirul Quran
Maulana Wahiduddin KhanOn ayahs 49–52
The Quraysh of Makkah considered themselves adherents of the Truth and the companions of the Prophet adherents of falsehood. They were so certain about this, that they prayed standing before the Kabah, ‘O God! Give success to the one of these two parties which is in the right and destroy the party which is in the wrong.’ However this faith of theirs was false. This type of faith always develops due to glorification (taz’in) of wrong action. Satan taught the people of Makkah that they were the followers of the recognised, well-known prophets, Abraham and Ishmael, while the Muslims were the followers of a person whose very claim to being a prophet was controversial; they were the inheritors of the Ka‘bah, while the Muslims had been banished from the land of the Ka‘bah. Satan having created such ideas in the hearts of the Makkans, infused false faith in them. They thought that whatever they were doing was done rightly and that, at all events, God would come to their assistance. On the one hand, the Makkan opponents considered their false conviction to be true on the basis of the above-mentioned factors. On the other hand, when they saw that the Prophet’s companions had engaged themselves on the Islamic front with stronger conviction and the will to self-sacrifice, they set about trying to show that their true faith was unreliable by saying that it was merely religious fanaticism; that they were going mad under the influence of the eloquent utterances of the Prophet. But, when there was a clash between the two groups and God’s succour came down in favour of the Muslims, Satan fled, abandoning the opponents of Islam. By the Grace of God, the Muslims’ hearts became strengthened. God certainly helps those who rely on Him. But, God’s help appears only when believers prove their faith in God to such a great extent that those who deny the truth feel justified in saying that they (the Faithful) have gone mad.