ה֣וֹי הָאֹמְרִ֥ים לָרַ֛ע ט֖וֹב וְלַטּ֣וֹב רָ֑ע שָׂמִ֨ים חֹ֤שֶׁךְ לְאוֹר֙ וְא֣וֹר לְחֹ֔שֶׁךְ שָׂמִ֥ים מַ֛ר לְמָת֖וֹק וּמָת֥וֹק לְמָֽר׃ (ס)
װײ צו די װאָס רופֿן דעם שלעכטן גוט, און דעם גוטן שלעכט; װאָס מאַכן פֿינצטערניש פֿאַר ליכט, און ליכט פֿאַר פֿינצטערניש; װאָס מאַכן ביטער פֿאַר זיס, און זיס פֿאַר ביטער.
Or HaChaim
3) They were to realise the difference between a serpent which crawls on earth, which is the lowest of the low and brings death in its wake, and the serpent constructed by Moses and placed high on a pole which brought life in that it revived people who had been bitten by the deadly poison of a snake. This should remind the people that they had spoken out against the mannah and had professed their preference for bread grown in the ground. Concerning such comments the prophet Isaiah exclaimed in Isaiah 5,20 "Ah, all those who call evil good and good evil, etc."