I'm not sure about the exact reason f0ro the bad blood, but Rantine views Orthus as a monster. He wants to civilize his people, while Orthus wants to lead them much deeper into barbarism. Rantine tried to make peace with the Bannor (I'm not sure if this peace involved an alliance against Orthus or just a ceasefire), but the Bannor betrayed his trusts and slaughtered all their emissaries and a orkish village too I think. I believe that Jonas then banished Rantine, who was not accepted among his brethren again until he returned with their Queen.
We shouldn't think that Rantine is exactly a good guy though, as he is a loyal servant of the evil goddess Bhall, who spoke to him in dreams to lead him to Sheelba.
We really don't know much about Orthus. We don't know who he is, or how he got the staff. I like to think that he is a relative of Capria, maybe a cousin thrice removed, but there is no proof that his ancestors were Bannor at all, as men of nations worshiped Bhall and were corrupted by her fall.
I have no real basis for this, but I'm starting to suspect that Rantine might be a descendant of the old priesthood whose family continued to guard such relics as her holy staff until Orthus, an impious descendant of the old monarchy or nobility, randomly decided to murder Rantine's entire family and to steal the relics.