The poem spells it out in several ways through its monsters. Internecine problems are very destructive. Unferth is a kin killer.
Intrafamily conflict is taboo.
Destructive lone wolves such as the Grendels are particularly bad. They are unguided missiles, out of control berserkers. They live beyond habitation and the firelight. Warrior who can't bond with anyone are dangerous. Remember this is an armed society that measures success in battle.