A helpful clue further to what is stated in Revelation 9:13-15 is given in chapter 16, where the River Euphrates is mentioned again. But before looking at that, it is more than a little helpful to grasp that the whole book of Revelation has 7 sections. Your text is in section 3, while the one I mention is in section 5. But all the sections deal with the same matters, adding further information as to what has been portrayed in the previous sections. As explained in this book (from where I get my answer to this questions):
"Like seven parallel (but not equal) tracks traversing the side of a
mountain, each appearing at one extremity and passing from sight at
the other, one above another in parallel, yet varying in length
proportionately as the mountain increases in height to the topmost
peak, so appears the Book of Revelation.
It follows that each ascending traverse affords a different view,
yields a distinctive perspective, presents a new aspect, yet of the
same basic journey in principle from one extremity of departure to the
other. But as the elevation heightens, so the journey decreases, and
the heavenly vision unfolds with the more astonishing clarity, till at
last all lies revealed before and beneath the spiritual gaze of the
discerning reader.
The lowest traverse gives the greatest earthly detail, whereas the
highest, wellnigh in the clouds of heaven, present a glimpse of that
vast, panoramic sweep of time and space from the vantage of the
heavenly places. This suddenly makes everything clear and in an
instant sets all in perspective.
But from foothills to peak, in each ascending parallel traverse, there
is the same extremity of departure relative to the mountainside, and
the same relative destination at its opposite point." The Revelation
of Jesus Christ, John Metcalfe, pp.53-54
http://www.johnmetcalfepublishingtrust.co.uk/contact_us.htm
As the text in question is in section 3 and the one I point to is in section 5, this answer cannot be exhaustive. Yet the twice mentioned 'river Euphrates' may help a little.
First, chapter 9 specifies that when those 4 angels are released, they "slay the third part of men". From the same book:
"The figure of a battle has been used repetitively towards the end of
various preceding parallel openings in the Book of the Revelation of
Jesus Christ, and therefore it is mo surprise to find the most
climactic imagery of all battled taken from the visionary figures of
the prophet Ezekiel and applied in the seventh and last parallel
division of the book. The allegorical concept of a great battle, with
two hundred thousand thousand fearsome horsemen, appeared under a
figure of dramatic vividness with the sounding of the sixth trumpet,
chapter 9:13-21, a prelude to the last trump. Metaphorical use of the
type of the land of Israel, and of the Euphrates, filled out this
passage." (Ibid p.567)
Note how chapter 9 details how this third part of men are killed: by the fire, and by the smoke, and by the brimstone which issued out of the mouths of the horses (verse 18). The 4 angels do not kill those men. Now comes proof of whether those men killed, and those who remain alive, are 'believers' or not.
"And the rest of the men which were not killed by these plagues yet
repented not of the works of their hands, that they should not worship
devils, and idols... Neither repented they of their murders, nor of
their sorceries, nor of their fornication, nor of their thefts."
Revelation 9:18-21 K.J.V.
Those who are killed, and those who remain, were, and are, evildoers in God's sight. All of them were, and are, unrepentant. All who were, and are, God's people, have repented and are saved by grace, whatever happens on Earth. God's angels assist them during the tribulations symbolically depicted in Revelation, while demons influence unrepentant ones to carry out all that list of evil in verse 20.
"But in chapter 16 this falls upon the present heavens and earth, and
upon men whilst still living in the world. Suddenly it shall fall.
'And the sixth angel poured out his vial upon the great river
Euphrates; and the water thereof was dried up, that the way of the
kings of the east might be prepared.' ...
"How aptly does the previous description of the sounding of the sixth
angel, with its dire consequences, chapter 9:13-21, fit the exposition
of chapter 16:12-14, and verse 16 [about the final battle of
Armageddon]...
"Whereas the sounding of the sixth trumpet resulted in the loosing of
the four angels bound in the great river Euphrates, at the last day
the pouring out of the vial of the sixth angel upon the great river
Euphrates causes the waters to dry up, Revelation 16:12. Clearly this
is the culmination of the vision previously unfolded, Revelation
9:14." (Ibid pp.442-3)
The book then goes on to deal with the symbolic significance of this river Euphrates - far too much to copy here. The subject is massive, which is why the book takes over 600 pages to detail those seven sections in Revelation, then link them together for the finale.
For the purpose of this question, the answer is that the 4 angels bound in the symbolic river Euphrates may or may not be "fallen", but they do not kill anyone. The symbolic horses they muster do the killing. Those 4 angels are under God's control, bound until a certain time when he commands their release for a particular purpose. And all who die are unrepentant sinners. They are not God's people. It is the beast out of the sea (given its power by the dragon) that makes war with the saints on Earth, to overcome them. See chapter 13 verses 1-8.
