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.