John Gill Commentary Revelation 8:11

"and the name of the star is called Wormwood: and the third part of the waters became wormwood; and many men died of the waters, because they were made bitter." — Revelation 8:11 (ASV)

And the name of the star is called Wormwood
Because of the bitter afflictions, sorrows, and distresses which it was the instrument of; just as Naomi called herself Mara, because the Almighty had dealt bitterly with her, (Ruth 1:20) ;

and the third part of the waters became wormwood ;
that is, the inhabitants of the provinces and cities belonging to the Roman empire were afflicted with grievous and bitter afflictions and calamities; so great distresses are called wormwood, and waters of gall given to drink, (Jeremiah 9:15) (Lamentations 3:19) ;

and many men died of the waters, because they were bitter ;
through the barbarities and cruelties of these savage people, who afflicted the empire: there seems to be an allusion to (Exodus 15:23) .