John Gill Commentary Revelation 16:7

"And I heard the altar saying, Yea, O Lord God, the Almighty, true and righteous are thy judgments." — Revelation 16:7 (ASV)

And I heard another out of the altar say
That is, another angel that came out of the altar; see (Revelation 14:18) and who represents the souls under the altar, whose blood had been shed by the above persons; compare with this (Revelation 6:9) though, there, such whose blood had been shed by Rome Pagan are described; the Ethiopic version calls this angel as before, "the angel of the fountains of water"; and the Alexandrian copy, and Syriac and Arabic versions, read, "I heard the altar saying": as follows,

even so, Lord God Almighty, true and righteous are your judgments ;
the same as in (Revelation 15:3) (19:2) . This angel joins the other, and approves and confirms what he says; applauding the judgments of Christ upon the worshippers of the beast, from the verity of them, being what were threatened, and from the justice of them, being what they deserved.