Christ is far greater than the angels. And of the angels he saith, Who maketh his angels spirits, and his ministers a flame of fire. Angels are spirits whereas believers are flesh and blood. And they're made so by Christ. So Christ is the Creator of the angels.
Fire has to do with the altar and judgment.
Compare verse 7 with verse 14. No angel is a son. Christ is God. Christ is utterly superior over the angels and everything. Wordly kings many times do not have a righteous sceptre as does Christ.