And THE REST WERE KILLED with the SWORD which came from the mouth of him who sat upon the horse,
Many biblical scholars believe that 'THE REST' refers to the kings and their armies in verse 19. This may be the case, however, this really doesn't tell us anything since we do not know who is in these armies. We know that those who took the mark of the beast are still left on this earth, but whether they all are in the armies isn't stated. Traditionally, armies have only been composed of men (Deuteronomy 20).
If these are spiritual armies, then we can assume that this refers to the rest of evil or the demonic world. This may be what the odd statement Jesus used in Luke 21:25-26-28, 'the powers of the heavens' refers to. Most of us have so little experience in this realm. In the Old Testament, Jeremiah uses the phrase 'inhabitants of the earth' (Jeremiah 25:29-33) in his prophesy. The 'inhabitants of the earth' may well include these evil entities (Revelation 12:9).
It shouldn't surprise anyone that just the SWORD or a word of TRUTH from the mouth of him who sat upon the horse can kill 'THE REST'. That is it's intended purpose (Revelation 19:15). Even while Jesus was on this earth, it only took a word:
and all the birds were filled with their flesh.
This probably is both literal and symbolic. Physically, so many will die at the swipe of the SWORD that there is nobody to bury all of them, and they will lie as dung on the ground (Zephaniah 1:14-17-18).
Symbolically, the birds are a kind of cleansing force which will remove (or become filled with) evil (flesh). We don't understand this process very well for it implies that these birds ultimately are 'sacrificed' into the lake of fire. That there is a correlation is supported by the prominent use of birds in the purification ritual for leprosy (Leviticus 14:1-4-9). The result, however, is to return the earth to its pre-evil state (Genesis 2:4-14) such that the rightful owner may inhabit it (Revelation 20:4-6).