Psalm 110 is a Messianic psalm, a prophetic psalm of the Messiah coming to rule and reign. Messiah will not only deliver Israel from her enemies, as the judges did, He will also execute kings of the nations in the day of His wrath. And Messiah will also be high priest forever, after the order of Melchizedek, not Aaron.
Melchizedek was the King of Salem (Jerusalem) who met Abraham on his return from the slaughter of the four kings, with bread and wine (Gen 14:18-20). And Abraham gave him a tithe of all he had gained in the victory. His name also means, in Hebrew, King of Righteousness. Hebrews 7 talks extensively about what Messiah being a priest forever according to the order of Melchizedek means.
The thing that we have to remember, is that Torah is a book of history – prophecy: in fact, both Melchizedek and Aaron are types, pictures, object lessons, prophesying about the Messiah and what He would do and be when He came. We cannot try to “fit” Jesus into Aaron’s shoes, or Melchizedek’s shoes, or anyone else’s shoes. God used these historical people to rather, paint a picture of Messiah. They loosely fit into His shoes for a time and a season. But Messiah is God, He is king, He is creator, He is priest, He is savior, and He will be worshiped as God and King forever and forever.