This is about the Jews who had killed over forty, or reportedly hundred seventy, prophets. Moreover, they had disbelieved in all the attributes of Prophet Muhammad (saws) that are mentioned in the Torah.
(… وَيَقْتُلُونَ النَّبِيِّينَ بِغَيْرِ حَقٍّ …) (…and kill the prophets without right..): Whenever the Almighty talks about killing in general, and prophets in particular, He mentions ‘without right.’ The killing of prophets can never be righteous. However, Allah confirms the fact that killing is always without a right in order not to make it acceptable in any society.
(… وَيَقْتُلُونَ الَّذِينَ يَأْمُرُونَ بِالْقِسْطِ مِنَ النَّاسِ …) (…and kill those who order justice from among the people …): They kill them simply because they call for justice among people.
( فَبَشِّرْهُم بِعَذَابٍ أَلِيمٍ … ) (…give them tidings of a painful punishment): Warn the killers of a severe punishment.