Set difference from two files in unix

That's typically what join is for:

join -v2 setA setB

That's joining the files on the first column (which is assumed to be sorted lexically), and print the rows of the second file which are not paired.

awk 'NR==FNR {key[$1]; next} !($1 in key)' setA setB

1 123
3 def

That reads the first file and stores the "ids" in the array "key", then it only prints from the 2nd file if the id is not in the key array.