This second woe, like the first, is proclaimed with a sense of sadness; but, there is an emphasis, a declaration of condemnation,
with the sense of divine judgment of the evil upon the disbelieving world.
The first woe is past; behold, two woes are still coming after these things.
(Rev 9:12)
The angel of the sixth trumpet releases the four bound angels at the Euphrates River who are limited to kill 1/3
of the population. In consideration of the fourth seal, in which 25% of the population was killed, the death of 1/3 of the surviving
75% leaves at most 50% of the original population before the tribulation began (Rev 9:13-19).
Note that "the rest of mankind, who were not killed by these plagues, did not repent of the works of their hands...
and they did not repent of their murders nor of their sorceries nor of their immorality nor of their thefts"
(Rev 9:20-21).