Matthew's gospel only includes angels at the beginning (1:20-21, 2:13-15, 2:19-20) and at the end (28:1-8). Especially in chapters 1 and 28, the angels tell people not to be afraid. They tell Joseph not to be afraid to take Mary as his wife and they tell the two Marys not to be afraid at the empty tomb. In and around the same passages of angelic activity Matthew records two others who are afraid, but who aren't encouraged by the angels. Early on, Herod and "all Jerusalem" were frightened (2:3) and later on, the guards at the tomb "shook and became like dead men" (28:4).
Herod, the guards and all Jerusalem are afraid but get no consolation. Who do they represent? It's probably an easy fit to say that they represent empire; the machinations of humanity to which God says an eternal "No!" In a way, that's consistent with the broader content of the gospels. Jesus is the messiah who poses a challenge to Rome, to the Jewish populace and to collaborators with both of those authorities. The gospel of Matthew has a mission for Jesus, and standing in the way of that mission are (amongst others) Herod, all Jerusalem and the tomb guards. When God intervenes it provokes fear. The actions of God, mediated through the activities of the angels, impose on their position in this world.
Matthew's lesson for us is about being in the kingdoms of this world or being in the kingdom of God. When God moves, allegiance to this world will bring no comfort whereas allegiance to the basilea theou immediately consoles. God's actions are bold and terrifying, but only for those enmeshed in the kingdoms of this world.