Revelation 6:3

The next three seals give us a sad prospect of great and desolating judgments with which God punishes those who either refuse or abuse the everlasting gospel. Though some understand them of the persecutions that befell the church of Christ, and others of the destruction of the Jews, they rather seem more generally to represent God's terrible judgments, by which he avenges the quarrel of his covenant upon those who make light of it. I. Upon opening the second seal, to which John was called to attend, another horse appears, of a different colour from the former, a red horse, Rev 6:4. This signifies the desolating judgment of war; he that sat upon this red horse had power to take peace from the earth, and that the inhabitants of the earth should kill one another. Who this was that sat upon the red horse, whether Christ himself, as Lord of hosts, or the instruments that he raised up to conduct the war, is not clear; but this is certain, 1. That those who will not submit to the bow of the gospel must expect to be cut in sunder by the sword of divine justice. 2. That Jesus Christ rules and commands, not only in the kingdom of grace, but of providence. And, 3. That the sword of war is a dreadful judgment; it takes away peace from the earth, one of the greatest blessings, and it puts men upon killing one another.

— Matthew Henry's Concise Commentary (public domain)

and see--omitted in the three oldest manuscripts, A, B, C, and Vulgate.

— Jamieson-Fausset-Brown Commentary (public domain)