They seem to be very aggresive creatures that have no problems with killing humans. Furthermore, they are to some degree arrogant to think that they can kill anything as demonstrated when old spig thinks he can kill alice. This might be due to them being very secure of their power.
Brain Guzzlers eat the brain of their victims so that they then can jump into their body. When they do jump into a humans body they use them as a human vessel. Many witches use this to retrieve corpses, so that they can cut off their thumb bones for use in their magic (Bone Magic).
- Salt (Alice used this to kill Old Spig).
- By cutting off the limbs of a brain guzzler, they can be slowed down.