[3:21] Those killed those who advocated justice among the people, promise them a painful retribution.
[3:22] Their works have been nullified, both in this life and in the Hereafter, and they will have no helpers.
Luke 11: 47 – 51
47 “Woe to you, because you build tombs for the prophets, and it was your ancestors who killed them. 48 So you testify that you approve of what your ancestors did; they killed the prophets, and you build their tombs. 49 Because of this, God in his wisdom said, ‘I will send them prophets and apostles, some of whom they will kill and others they will persecute.’ 50 Therefore this generation will be held responsible for the blood of all the prophets that has been shed since the beginning of the world, 51 from the blood of Abel to the blood of Zechariah, who was killed between the altar and the sanctuary. Yes, I tell you, this generation will be held responsible for it all.
Other messengers that were killed were: John, Jesus and Rashad.
Interesting that it also states “those who advocated justice among the people.” This sounds like it could mean people who were not messengers or prophets like Martin Luther King.