Then said he to Zebah and Zalmunna, What manner of men were they whom you killed at Tabor? They answered, As you are, so were they; each one resembled the children of a king.
Gideon, as the "de facto" head of state, was the appropriate center of authority for the execution of all murderers.  In this connection, it is of interest that some scholars apparently do not know the difference between "Thou shalt do no murder," which is the proper translation of  Exodus 7 and, "The man shall surely be put to death"  (Numbers 15:35), which was also the Word of God concerning certain violators of Divine law, including murderers.  (For further comment on this, see Vol. 1 (Genesis) of the series on the Pentateuch, pp. 136,137.)