Those who disbelieve in the signs of Allāh and kill the prophets without right and kill those who order justice from among the people - give them tidings of a painful punishment.
Chastising the Jews for Their Disbelief and for Killing the Prophets and Righteous People
This Ayah chastises the People of the Book for the transgression and prohibitions they committed by their denials in the past and more recent times, of Allah's Ayat and the Messengers. They did this due to their defiance and rejection of the Messengers, denial of the truth and refusal to follow it. They also killed many Prophets when they conveyed to them what Allah legislated for them, without cause or criminal behavior committed by these Prophets, for they only called them to the truth,
(And kill those men who order just dealings) thus, demonstrating the worst type of arrogance. Indeed, the Prophet said,
«الْكِبْرُ بَطَرُ الْحَقِّ وَغَمْطُ النَّاس»
(Kibr (arrogance) is refusing the truth and degrading people)
This is why when they rejected the truth and acted arrogantly towards the creation, Allah punished them with humiliation and disgrace in this life, and humiliating torment in the Hereafter. Allah said,
فَبَشِّرْهُمْ بِعَذَابٍ أَلِيمٍ
(then announce to them a painful torment) meaning, painful and humiliating,
(They are those whose works will be lost in this world and in the Hereafter, and they will have no helpers. ).
Ma'arif al-Qur'an
Mufti Muhammad ShafiOn ayahs 21–22
In the earlier part of the Surah, the text mostly beamed at the Christians. In verse 20, 'those who have been given the Book' includes both Christians and Jews. Now, verses 21-22 here, talk about some of the unusual doings of Jews. Ruh al-Ma'ani while commenting on this verse reports a hadith from the Holy Prophet ﷺ as narrated by Ibn Abi Hatim ؓ . While explaining this verse, he said that Bani Isra'i1 slew forty three prophets at one and the same time. One hundred and seventy pious, elders stood up asking them to uphold justice. They slew them as well on the same day. (Bayan a1-Qura'n)
In verse 21, 'those who disbelieve the verses of Allah' refers to Jews who did not believe in the Injil انجیل and the Qur'an. 'Slay the Prophets (علیہم السلام) unjustly means that they know that they are doing so without justice. 'Those who bid justice' are people who teach moderation in deeds and morals.
Because of this whole set of their terrible deeds, verse 22 says that all their good deeds have gone waste both here and there, and when they are punished, they will find no one to assist them.
Tazkirul Quran
Maulana Wahiduddin KhanOn ayahs 19–21
The God of the universe is the One and only God and He likes justice. This assertion is made in all the revealed scriptures which are still in their original form. The vast universe run by its Master through His unseen agents, the angels, is exactly what it ought to be and as perfect as it can be. According to established knowledge, the universe seems to be a completely unitary system. This would indicate that it has a single Planner. Moreover, everything in the universe being in its right place is a proof that its Lord loves justice and righteousness rather than injustice and wrongdoing. Then how can God, who has maintained justice in the vaster universe, allow injustice in human matters? Each and every part of the universe is in a complete state of ‘surrender’, that is, all its functions are performed in accordance with the appointed plan of God. Exactly the same performance is required of man. Man must recognize his Lord and mould his life in accordance with God’s plan. Making anyone else, save God, the centre of his attention or nurturing the thought that God’s judgement can be based on anything other than justice is inconceivable in the present universe.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.