They are all in a sense, Dutch.
Colm the Outlaw represents Dutch's pettiness. Colm takes it personally against Dutch but makes Arthur see Dutch puts personal vendettas over the wellbeing of his gang.
Cornwall the Magnate represents Dutch's greed. Cornwall is all about money and makes Arthur see Dutch putting money before his gang.
Bronte the Mobster represents Dutch's arrogance. Bronte underestimates Dutch and treats him as expendable, but makes Arthur see that Dutch overestimates himself and isn't as smart as he thinks he is.
Fussar the Tyrant represents Dutch's cruelty. Fussar is cruel and power-hungry but shows Arthur that Dutch is much the same.
Milton the Authority represents Dutch's ruthlessness. Milton shows Arthur that to Dutch, the ends justify the means. He also showcases Dutch's fear and weakness.
All of the antagonists in the game show that despite all of his fancy talk of idealism, Dutch is no different from any of the people he hates.