If an angel had been sent down, then he would be in the form of a human being, as Allah (swt) says:

 وَلَوْ جَعَلْنَاهُ مَلَكًا لَّجَعَلْنَاهُ رَجُلًا وَلَلَبَسْنَا عَلَيْهِم مَّا يَلْبِسُونَ

(And if We had made him an angel, We would have made him [appear as] a man, and We would have covered them with that in which they cover themselves.) (Al-An’aam: 9)

(… وَلَوْ أَنزَلْنَا مَلَكًا لَّقُضِيَ الْأَمْرُ… ) (…But if We had sent down an angel, the matter would have been decided; …): If Allah (swt) had willed that mankind would submit and believe, the matter would be settled, and He would destroy them when they saw they proofs and miracles but did not believe in them. Even if He sent down an angel, they would then have doubts about him.

(  ثُمَّ لَا يُنظَرُونَ… ) (…then they would not be reprieved): They would not be given more time, but instead, punishment would come upon them.