Ultimate power rests with God who allows the judgment for His purposes. See commentary on Revelation 6:2.
take peace from the earth
Although the rider on the white horse appears to conquer by means of negotiation and avoids bloodshed, this does not last. Eventually he shows his true colors as the second rider joins the ridebringing war and death. This may correspond to the point in Daniels vision where another horn which came up, before which three fell, namely, that horn which had eyes and a mouth which spoke pompous words, whose appearance was greater than his fellows (Dan. Dan. 7:20). It appears that the little horn may only progress so far by means of peace before he is forced to go to war and assert his position with violence. Shortly after this point: I was watching; and the same horn was making war against the saints, and prevailing against them (Dan. Dan. 7:21 cf. Rev. Rev. 6:9-10+). This is the time of the beginning of sorrows which Jesus spoke of (Mtt. Mat. 24:6-8). See The Synoptic Gospels.
should kill one another
σφάξουσιν [sphaxousin] : To slaughter . . . Of the killing of a person by violence . . . butcher or murder someone2 . The same root described the Lamb as though it had been slain (Rev. Rev. 5:6+). This will be a time of unprecedented bloodshed which will forever puncture the naive view of humanism that man is inherently good.
a great sword
This sword is wielded by the second horseman, but it is sent by God:
For thus says the LORD God of Israel to me: Take this wine cup of fury from My hand, and cause all the nations, to whom I send you, to drink it. And they will drink and stagger and go mad because of the sword that I will send among them. Then I took the cup from the LORDS hand, and made all the nations drink, to whom the LORD had sent me. (Jer. Jer. 25:15-17)This is the pattern of Gods judgment to nations who reject HimHe makes them mad and turn upon one another (Eze. Eze. 38:21). While Scripture does not give the details, the advances in modern weaponry suggest a terrible, unimaginable holocaust.3
2 Frederick William Danker and Walter Bauer, A Greek-English Lexicon of the New Testament and Other Early Christian Literature (Chicago, IL: University of Chicago Press, 2000), 796.
3 John MacArthur, Revelation 1-11 : The MacArthur New Testament Commentary (Chicago, IL: Moody Press, 1999), Rev. 6:4.