Unlike player villages, the morale for attacking barbarian villages is always set at 100%. You can take over barbarian villages just like any other village. Although barbarian villages have no owner, depending on the world they will grow to a certain size.
When players are deleted due to an infraction of policy, inactivity, or deletion of account, their villages turn into barbarian villages. Sometimes units are placed in barbarian villages that formerly belonged to an infracted player, so they can be dangerous.