Some Scripture passages that speak of the destroying angel/s:
Ex 12:23 For the LORD will pass through to slay the Egyptians; and when he sees the blood on the lintel and on the two doorposts, the LORD will pass over the door, and will not allow the destroyer to enter your houses to slay you.
1 Chronicles 21:15 And God sent the angel to Jerusalem to destroy it; but when he was about to destroy it, the LORD saw, and he repented of the evil; and he said to the destroying angel, “It is enough; now stay your hand.”
Psalm 78:49 He let loose on them his fierce anger, wrath, indignation, and distress, a company of destroying angels
My thoughts are that it is the fallen angels - Satan and/or the others who rebelled against God. It is God who restrains their malevolent actions towards us, but there are times when He allows them to exercise it. Read for example
Job 1:6-12 Now there was a day when the sons of God came to present themselves before the LORD, and Satan also came among them…
9 Then Satan answered the LORD, “Does Job fear God for nought?
10. Hast thou not put a hedge about him and his house and all that he has, on every side? Thou hast blessed the work of his hands, and his possessions have increased in the land.
11 But put forth thy hand now, and touch all that he has, and he will curse thee to thy face.”
12 And the LORD said to Satan, “Behold, all that he has is in your power; only upon himself do not put forth your hand.” So Satan went forth from the presence of the LORD.
A couple other questions are if this is a fallen angel that God still uses or one still loyal to God.
Probably. I know there are saints who have personally had to confront Satan – not because they sought contact with the devil or because of some personal sin of theirs.
Consider also those who experience demonic possession. That is usually a result of the person freely choosing to draw too near to Satan thru occult involvement, and God sometimes allows them to experience the one/s whom they have chosen.
The last question is more of an asumption of what would be the purpose of this Angel when we have all as a race reached our final judgement?
They will be confined permanently to hell and will not be able to torment in any way those who are in heaven with God.