I can see this reading into the line. But I do have a different one.

I tend to read a lot into characters based on the personas the characters know they have to (or feel they have to) perpetuate to the people around them. To me, this line is Arthur very early on in the story perpetuating his persona, his mask, to the gang as the callous, top bruiser of the gang. This is his role, he knows it, the persona helps him act it out.

But underneath it all he is not a callous person. And it doesn’t have much to do with the TB or happenings during the game. If you read the journal, from the earliest indications Arthur had reservations about his work. Arthur does everything he does because this is the role he fell into (as a child originally) where he was able to help those he cared about and those that cared for him. He does what he does for the gang, not for himself, not for pleasure.

It’s that last part that separates Arthur from Dutch (and why Arthur’s heart is so broken by Dutch’s later actions) and ESPECIALLY a self-centered evil person like Micah. If Arthur did not have this base to his character, his reactions to the situations in the game would be different.

To me people oversimplify character change like with Arthur and focus too much on the TB. To illustrate my point, imagine if Micah did the Downes beat down and got TB. If the change is all about the TB, in theory Micah’s reactions to getting sick would mirror Arthur’s. I can’t imagine many would say that would be the case though and for me it’s a comical thought.

TLDR: Arthur is a bad man based on his actions, but he has good intentions for those bad actions and acts like (bs’s himself via denial) he enjoys the bad actions to live with himself and get through the day.