Barbarism
Brutality
Barbarians go around and commit acts of barbarism! What else would they do? They're barbarians!
ばんこう
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
あいつらはそこで蛮行を繰り返しているんだ。
They repeated the brutality over there.