I'm not an expert on this so I wouldn't say he mass murdered clans (Killing innocents) and stole their cattle. It's only British colonial sources claiming him to be some sort of monster, I don't believe their claims. He was a rebel so there obviously going to lie about him and spread all sorts of deceptive propaganda.
He probably only killed enemies on the battlefield and when he defeated them, all their battle possessions belonged to the dervishes. This is something normal in war, when you defeat someone, you take their tanks, Guns, camels, swords and everything they had on the battlefield.