The rider of the second horse is often taken to represent War (he is often pictured holding a sword upwards as though ready for battle) or mass slaughter. His horse’s color is red; and in some translations, the color is specifically a “fiery” red. The color red, as well as the rider’s possession of a great sword, suggests blood that is to be spilled. The sword held upward by the second Horseman may represent war or a declaration of war, as seen in heraldry. In military symbolism, swords held upward, especially crossed swords held upward, signify war and entering into battle. (See for example the coat of arms, of Jeanne of Arc.)
The second Horseman may represent civil war as opposed to the war of conquest that the first Horseman is sometimes said to bring. Other commentators have suggested that it might also represent the persecution of Christians.
According to Edward Bishop Elliott’s interpretation of the Four Horsemen as symbolic prophecy of the history of the Roman Empire, the second seal is opened and the Roman nation that experienced joy, prosperity and triumph is made subject to the red horse which depicts war and bloodshed — civil war. Peace left the Roman Earth resulting in the killing of one another as insurrection crept into and permeated the Empire beginning shortly into the reign of the Emperor Commodus.
Elliott points out that Commodus, who had nothing to wish and everything to enjoy, that beloved son of Marcus Aurelius who ascended the throne with neither competitor to remove nor enemies to punish, became the slave of his attendants who gradually corrupted his mind. His cruelty degenerated into habit and became the ruling passion of his soul.
Elliott further recites that, after the death of Commodus, a most turbulent period lasting 92 years unfolded during which time 32 emperors and 27 pretenders to the Empire hurled each other from the throne by incessant civil warfare. The sword was a natural, universal badge among the Romans, of the military profession. The apocalyptic figure indicated by the great sword indicated an undue authority and unnatural use of it. Military men in power, whose vocation was war and weapon the sword, rose by it and also fell. The unrestrained military, no longer subject to the Senate, transformed the Empire into a system of pure military despotism. (Source: Wikipedia)