How (will it be) when the angels will take their souls at death, striking their faces and their backs?
That is, how their situation will be when the angels come to take their lives, and their souls cling to their bodies, causing the angels to extract them by force, harshness, and beating.
This is similar to Allah's saying,
وَلَوْ تَرَى إِذْ يَتَوَفَّى الَّذِينَ كَفَرُواْ الْمَلَـيِكَةُ يَضْرِبُونَ وُجُوهَهُمْ وَأَدْبَـرَهُمْ
If you could but see when the angels take away the lives of the disbelievers, striking their faces and their backs. (8:50)
And His saying,
وَلَوْ تَرَى إِذِ الظَّـلِمُونَ فِى غَمَرَاتِ الْمَوْتِ وَالْمَلَـيِكَةُ بَاسِطُواْ أَيْدِيهِمْ
If you could but see when the wrongdoers are in the agonies of death, while the angels extend their hands? (6:93)
meaning, to beat them.
أَخْرِجُواْ أَنفُسَكُمُ الْيَوْمَ تُجْزَوْنَ عَذَابَ الْهُونِ بِمَا كُنتُمْ تَقُولُونَ عَلَى اللَّهِ غَيْرَ الْحَقِّ وَكُنتُمْ عَنْ ءَايَـتِهِ تَسْتَكْبِرُونَ
(saying): "Deliver your souls! This day you will be recompensed with the torment of degradation because of what you used to utter against Allah other than the truth, and because you used to arrogantly reject His signs." (6:93)
Because of the above, Allah says,
ذَلِكَ بِأَنَّهُمُ اتَّبَعُوا مَا أَسْخَطَ اللَّهَ وَكَرِهُوا رِضْوَانَهُ فَأَحْبَطَ أَعْمَالَهُمْ