First man

The modern evil, we have said, greatly turns on this: that people do
not see that the exception proves the rule. Thus it may or may not be
right to kill a murderer; but it can only conceivably be right to kill
a murderer because it is wrong to kill a man. If the hangman, having
got his hand in, proceeded to hang friends and relatives to his taste
and fancy, he would (intellectually) unhang the first man, though the
first man might not think so.

Can someone paraphrase last sentence for me? Who is the first man? Adam?