قَالَ رَبِّ بِمَا أَنْعَمْتَ عَلَيَّ فَلَنْ أَكُونَ ظَهِيرًا لِّلْمُجْرِمِينَ (He [ Musa ] said, "Omy Lord, since You have favoured me, I will never be a supporter to the sinners." -Al-Qasas 28:17 ⧉). When Allah Ta’ ala pardoned this slip of Sayyidna Musa (علیہ السلام) he said in gratitude to Allah Ta` ala that he would never again help a criminal. The second incident of the quarrel had proved that the Isra'ili in whose defense Musa (علیہ السلام) had involved himself in the brawl was quarrelsome. Therefore, by declaring him a criminal he vowed not to help such a person again. Sayyidna Ibn ` Abbas ؓ while explaining this verse Al-Qasas 28:17 ⧉, has maintained that the word 'criminals' stands here for infidels. Qatadah has also interpreted the verse in almost the same way. On the basis of these commentaries it seems that actually the Isra'ili, whom Sayyidna Musa (علیہ السلام) had defended, was not a Muslim but he helped him considering that he was an oppressed person.
Rulings
It is worth noting here that this saying of Musa (علیہ السلام) has established two rulings about two issues. One, that even if an oppressed person is a sinner or infidel, he should be helped. The second ruling is that it is not permitted to help any criminal or oppressor. Scholars have argued on the basis of this verse Al-Qasas 28:17 ⧉ that working in the employment of cruel rulers is also not permitted, for they too would be regarded as their associates. Many citations from the righteous salaf (elders) have also been quoted in support of this argument. (Ruh u1-Ma’ ani) To provide support to the oppressors or to the infidels may take different forms. The different rulings about these forms are laid down in the books of fiqh in detail. This humble writer has also dealt with the subject in his Arabic book Ahkamul-Qur'an with greater elaboration. The scholars interested in the subject may consult it.