Adam Clarke on 1 Samuel 15:33

As thy sword hath made women childless - It appears that Agag had forfeited his life by his own personal transgressions, and that his death now was the retribution of his cruelties.

And Samuel hewed Agag in pieces -

1. What Samuel did here he did in his magisterial capacity; and,

2. It is not likely he did it by his own sword, but by that of an executioner. What kings, magistrates, and generals do, in an official way, by their subjects, servants, or soldiers, they are said to do themselves; qui facit per alterum, facit per se.