Four angels bound to the great river Euphrates

There were four angels bound up at the Euphrates River in Revelation, who were all set to kill a third of all the humans.  Apparently, they had 200,000,000 horsemen, who were ready to fulfill their purpose.  These angels were prepared for a very special time.  They were certainly fallen angels, since their sole purpose was to kill people.  They were released for one purpose, to kill millions of people.  The devil was also bound up in Revelation in the bottomless pit for a thousand years.  The Euphrates River was the farthest eastern boundary of the land promised to Israel.  The Euphrates River was connected to Babylon.  Are these fallen angels connected to Babylon?  Did they commit earlier sins for which they were bound?  We do not know.  What do you know about the Euphrates River?