In Half-Life 2, the Barnacle is an enemy that uses its tongue to pick the player up, and then bites them to death. I was wondering if NPCs such as citizens, Combine, zombies, and Alyx could be picked up and killed. I have seen them pick up boxes (and break them), headcrabs, and the like, but I have never seen any "people" get picked up. Is this possible to do?
Yes, barnacles can pick up anything that isn't bolted down or simply too heavy. As a general rule of thumb: if your gravity gun can pick it up, so can a barnacle. The only exception are living beings, which barnacles can easily eat, but the gravity gun can't pick up.
The first time this is shown is during Gordon's first encounter with barnacles, when a bird gets caught and eaten by one.
Over the course of the game, the player can then choose to kill barnacles on sight, or throw all sorts of objects at them for distraction, including barrels.
Unfortunately, there aren't many places where barnacles can actually eat NPCs, Combines, or zombies. Note that barnacles will kill zombies, but not eat them.Barnacle killing zombie in Anticitizen One. Barnacles eating headcrab and Combine in Anticitizen One.
Important NPCs, such as Alyx, cannot be eaten by barnacles; not without using cheats or console commands. Also, I've never seen friendly NPCs get caught by barnacles, only zombies or Combines.