wanton

Petty thefts, wanton assaults, purposeless outrage—to the man who held the clue all could be worked into one connected whole.

This, in truth, his neighbours might have pardoned, seeing that saints have never flourished in those parts, but there was in him a certain wanton and cruel humour which made his name a by-word through the West.

I remembered the case well, for it was one in which Holmes had taken an interest on account of the peculiar ferocity of the crime and the wanton brutality which had marked all the actions of the assassin.