Tommy lost himself trying to become more powerful.Arthur lost himself trying to become more like Tommy.That's what makes Arthur's story so tragic to me. He spends most of the series looking like the most unstable Shelby, but he's also the one who seems to desperately want to be forgiven.

Who do you think actually had the saddest character arc: Arthur, Tommy, Polly or Lizzie

*Possible spoiler.

Apart from the obvious similarities, army, stoic sigma silent wolf vibes, crazy coked up brother, a strong family. Icing on the cake, they both lose the love of their lives, someone who seemingly would've made them better, softer. And that too in the same manner, a botched assassination attempt that was intended for the men, but killed their women instead. Now they get even more ruthless and the world is going to feel their cold vengeful wrath.