Angels were originally another type of target in the Kill Devil game mode. Though they are currently not in the game, Angels were supposed to be the opposite of a Devil. Angels will pop up randomly around the map and after 3 seconds they will disappear. If a player kills an Angel the Angel will blow up, hurting everyone around it. Killing it gave the player no points. Angels were originally in the game mode, but were taken out soon after its release for an unknown reason.
It is best to leave these alone, or if an enemy is near it, try shooting it, as the area damage may kill the enemy.
Keep in mind that sometimes the explosion can go through walls