The angels themselves were not evil, but were used by God to do things we regard as awful, but which were obviously necessary. There are no such beings as angels who sin. The original language in which the Bible was written does not make it clear whether it is a heavenly or an earthly messenger that is being referred to. Occasionally the translators got it wrong. The angels are human messengers Korah, Dathan and Abiran.
Angels usually do their work without being seen by men.
When Elisha and his servant were being hunted by the king of Syria, Elisha knew God would help, but his servant was frightened. Elisha asked God to open his servant's eyes. Angels told Joseph, Mary and the shepherds about the birth of Jesus. They continued to guide Joseph by telling him to take the infant Jesus down to Egypt, and then he was told when it was safe to return. In the garden of Gethsemane, when Jesus knew his crucifixion was near, he prayed that he might do God's will.