There is only one God. So there is only one being in the whole world that can call himself God with the word "I".
Yet we have this malak Hashem repeatedly acting as if he is God himself.
Appearances of the "angel of the Lord" are often presented as theophanies: in Genesis 31:11–13 the angel of God says, "I am the God of Beth-el"; in Exodus 3:2–6 the angel of the Lord appeared to Moses in the flame of fire, and then the Lord says to him: "I am the God of thy father"; compare Genesis 22:11; Judges 6:11–22. At times the angel of the Lord speaks in such a way as to assume authority over previous promises (see Gen. 16:11 and 21:17). (source)
Let's take a look at that "I am the God of Bethel". Many things are problematic here.
One explanation is that the angel of Hashem is Hashem himself. If that's the case, we have problems. That's because the angel "appears" while Hashem can't be seen. Also that would suggest that Hashem incarnates as an angel.
Another is that the angel of Hashem is not Hashem. He should have said, "Hashem is the God of Bethel"
Anyway, with all those problems, who is that angel of the Lord? And why did he call himself the God of Bethel?
Note: this question is inspired by the boy Malak in the new Moses movie.