The dictionary defines evil as, morally wrong; immoral; wicked. It could be something harmful or injurious; the synonyms though may be the best descriptions.
Depending on how it is used evil can be; sinful, iniquitous, depraved, base, vile, nefarious, pernicious, destructive, wickedness, unrighteousness, or corruption, to name a few.
“Then I saw another angel flying in the midst of heaven, having the everlasting gospel to preach to those who dwell on the earth – to every nation, tribe, tongue, and people – saying with a loud voice, ‘Fear God and give glory to Him, for the hour of His judgment has come; and worship Him who made heaven and earth, the sea and springs of water.’” Revelation 14:6-7.
“I saw still another mighty angel coming down from heaven, clothed with a cloud. And a rainbow was on his head, his face was like the sun, and his feet like pillars of fire.” Revelation 10:1.
“He had a little book open in his hand. And he set his right foot on the sea and his left foot on the land…” Revelation 10:2.
“And to the angel of the church of the Laodiceans write, ‘These things says the Amen, the Faithful and True Witness, the Beginning of the creation of God: I know your works, that you are neither cold nor hot. I could wish you were cold or hot. So then, because you are lukewarm, and neither cold nor hot, I will spew you out of My mouth.’” Revelation 3:14-16.
