What does Judges 8:21 mean?

Zebah and Zalmunna said, 'Do it yourself. A man should die by a man’s hand, not a boy’s.' So Gideon killed them both and took the crescent ornaments from their camels’ necks. Judges 8:21 - Modern Text Bible

(The kings challenge Gideon to do it himself, and he does. He also takes their royal ornaments as trophies.)