The Swede is always eating whenever he interrogates Cullen. Why? Because he was starved near to death at Andersonville. It's as much about trying to regain the life he lost during the war as it is about intimidation.
The Swede hates being called the Swede and always insists that he is Norwegian. This gains added importance when you realize that Norway actually wasn't a sovereign country at the time of the series, and was unwillingly a province of Sweden. The Swede's anger at his nickname isn't just irritation with unworldly American laborers, it's nationalistic pride.
In addition, he was only ever referred to as 'Swede' by a Confederate soldier in Andersonville who frequently beat and humiliated him.
The Swede's hatred of Bohannon is initially motivated by the fact that he's a Confederate while the Swede is a former prisoner of theirs. Later, it is the many personal costs they suffer because of their rivalry that drive the hatred. It was initially just Evil Is Petty.
