`Ali bin Abi Talhah reported that Ibn `Abbas commented on the Ayah:
يحَسْرَةً عَلَى الْعِبَادِ
(Alas for mankind!), this means, woe to mankind! Qatadah said:
يحَسْرَةً عَلَى الْعِبَادِ
(Alas for mankind!) means, "Alas for mankind, who have neglected the command of Allah." The meaning is that they will feel regret and sorrow on the Day of Resurrection. When they see the punishment with their own eyes; they will regret how they disbelieved the Messengers of Allah and went against the commands of Allah, for they used to disbelieve in them in this world.
In verses 28 and 29, it was said: وَمَا أَنزَلْنَا عَلَىٰ قَوْمِهِ مِن بَعْدِهِ مِن جُندٍ مِّنَ السَّمَاءِ وَمَا كُنَّا مُنزِلِينَ إِن كَانَتْ إِلَّا صَيْحَةً وَاحِدَةً فَإِذَا هُمْ خَامِدُونَ " (And We did not send down to his people any army from the heavens after him, nor were We (in need ) to send down. It was no more than a single Cry, and in no time they were extinguished.)
Mentioned here is the Divine punishment that descended upon the people who had rejected the messengers and had beaten up Habib Najjar until he died a martyr. And regarding the sending of punishment, it was said that Allah did not have to send an army of angels to seize these people - nor was it the way of Allah to send such an army, because just a single angel of Allah is enough to destroy the greatest, mightiest and the bravest of nations. Why would he need to send an army of angels? After that, given there was a crisp description of the punishment coming upon them - it was just a single shrill Cry of the angel, and there they were, all extinguished under its sonic sweep. It appears in Hadith narrations that the archangel, Jibra'il al-'amin (علیہ السلام) ، holding the two sides of the city gate, came up with a hard and horrendous Cry, the shock from which proved unbearable for any living soul, and they all succumbed to sudden death.
The believer had risked his own life to support the call of the prophets before him. This action of his was of such great worth that he was sent to Paradise. After entering Paradise he did not speak ill of his people, although they were transgressors. On the contrary, he felt that, had they seen his reward, they would not have opposed the truth. This is the picture of a true believer. He is the well-wisher of others under all circumstances, even if he is oppressed by them.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.