Albert Barnes Commentary Judges 6:22

"And Gideon saw that he was the angel of Jehovah; and Gideon said, Alas, O Lord Jehovah! forasmuch as I have seen the angel of Jehovah face to face." — Judges 6:22 (ASV)

Alas, O Lord GOD! — Compare Joshua 7:7. because I have seen an angel of the Lord. Compare the marginal references, in which the notion that it was death for mortal man to see God appears clearly. The same notion prevailed among the pagan.