Why did Bolvar choose Thoras Trollbane to be one of his Four Horsemen?

I mean, I get Whitemane and Nazgrim (Being High Inquisitor and General of Garrosh's forces) and eventually Darion becoming the leader, but why did he choose Thoras given that he advocated for the slaughter of the Orcs and didn't do anything much noteworthy other than being killed by his son Galen. Despite worthier candidates like Thassarian or Koltira, why was Thoras chosen?