Angels are able to open locked doors

In Acts 5:19 we read that, in order to rescue Peter and John from prison, during the night an angel of the Lord opened the doors of the jail and brought them out. This suggests that angels have complete command of physical barriers. They cannot be chained or held back by any physical barrier. This is not surprise as they are essentially spiritual creatures.

Angels have always had their doubters and detractors

In a footnote in Acts 23:8 Luke tells us that The Sadducees say that there is no resurrection, and that there are neither angels nor spirits, but the Pharisees believe all these things. There have been sceptics in every age who deny the existence of angels. It is not unique to our own modern times. We ought not to be surprised then at unbelief. No doubt the angels themselves take a low view of their doubters while the demons chuckle to themselves at their success.

Angels were involved in the giving of the law

Acts 7:53 you who have received the law that was given through angels but have not obeyed it.

Galatians 3:19 Why, then, was the law given at all? It was added because of transgressions until the Seed to whom the promise referred had come. The law was given through angels and entrusted to a mediator.

Hebrews 2:2 For since the message spoken through angels was binding, and every violation and disobedience received its just punishment, ...

Deuteronomy 33:2 He said: "The LORD came from Sinai and dawned over them from Seir; he shone forth from Mount Paran. He came with myriads of holy ones from the south, from his mountain slopes.
Psalm 68:17 The chariots of God are tens of thousands and thousands of thousands; the Lord has come from Sinai into his sanctuary.

Matthew Henry comments on Acts 7:53 that the law is said to be received by the disposition of angels, because angels were employed in the solemnity of giving the law, in the thunderings and lightnings, and the sound of the trumpet. It is said to be ordained by angels (Gal. 3:19 ), God is said to come with ten thousand of his saints to give the law (Deu. 33:2 ), and it was a word spoken by angels, Heb. 2:2 . This put an honour both upon the law and the Lawgiver, and should increase our veneration for both. But those that thus received the law yet kept it not, but by making the golden calf broke it immediately in a capital instance.

Barnes notes that the phrase given through angels has been supposed to mean that the Law was given “amidst” the various ranks of angels, being present to witness its promulgation. Others suppose that the angels were employed as agents or instruments to communicate the Law. All that the expression fairly implies is that the Law was given amidst the attending ranks of angels, as if they were summoned to witness the pomp and ceremony of giving “law” to an entire people, and through them to an entire world. It should be added, moreover, that the Jews applied the word “angels” to any messengers of God; to fire, and tempest, and wind, etc. And all that Stephen means here may be to express the common Jewish opinion that God was attended on this occasion by the heavenly hosts, and by the symbols of his presence, fire, and smoke, and tempest.

Angels can destroy people

1 Chronicles 21:15 And God sent an angel to destroy Jerusalem. But as the angel was doing so, the LORD saw it and relented concerning the disaster and said to the angel who was destroying the people, "Enough! Withdraw your hand." The angel of the LORD was then standing at the threshing floor of Araunah the Jebusite

1 Corinthians 10:10 And do not grumble, as some of them did - and were killed by the destroying angel.

Exodus 12:23c and he will not permit the destroyer to enter your houses and strike you down.

Acts 12:23 Immediately, because Herod did not give praise to God, an angel of the Lord struck him down, and he was eaten by worms and died. 

More than once we hear of angels involved in the act of destroying people, killing them. It is not entirely clear whether such angels may be fallen or unfallen. This appears to be what happened in David's time, although given the reference to the angel of the LORD, it may be God himself who is in mind. Paul suggests in the New Testament that a destroying angel was responsible for putting people to death in the wilderness. The destroyer of Exodus 12 is popularly thought of as the angel of death and this may be the reference here too. In Acts Herod is killed by an angel who inflicts a terrible disease on him.