I'm gonna talk about episode 18 once again because holy fuck that episode sure does make me want to eat glass/pos
Ok so the argument in episode 18. It's Bad. It is a genuinely horrific argument that ended in physical violence, which sure is an accomplishment when you literally share a body with the person you are fighting with.
Now, most arguments that John and Arthur have, are eventually just swept under the rug, after all there's no point bickering when there is a creature trying to kill you to death right? But this one is not.
John broke Arthur's trust and Arthur does not let go of that. It takes him a while before he even gets on civil terms with John. He says that he'll only agree to work with John if John agrees to never mention Faroe's name ever again, which John agrees to.
A lot of their arguing and petty squabbling in episode 19 comes from Arthur still being upset from what John said in episode 18.
Episode 20 rolls around, and John and Arthur are back to being on good terms for the majority of the episode. The King however, brings up this argument to Arthur, and Arthur states that whilst he hates Join for betraying his trust, he doesn't hate John for calling him an unfit father.
Whilst that may be true, it doesn't mean it didn't hurt for his best friend to tell him that he sees Arthur as the absolute worst part of himself.
Anyway, we come to episodes 24 and 25 which is where things really get interesting in my opinion. Throughout episodes 21-23, Arthur has told Yellow about how he reminds him of the worst parts of John, the parts that betrayed his trust.
But now John is back. John is back and Arthur is overjoyed to have his friend back but things have happened whilst John has been gone, and now Arthur is dead set on killing a man that John knows nothing about. A man that Arthur simply describes as a bad person, meaning he deserves to die.
John is naturally shocked at this. Arthur is a man who has generally shown sympathy and kindness to all sorts of people and creatures when he's sure they're not a direct threat to his life.
What John doesn't know is that Larson cheerfully bragged to Arthur about willingly sacrificing his own daughter as she begged for him to save her. John does not know this because Arthur does not tell him this.
Of course he doesn't. Arthur has never been one to talk willingly about his own trauma, this on top of the way John betrayed his trust in episode 18? Of course he's not going to tell him.
The last thing Arthur wants at a time when he is already incredibly emotionally vulnerable is for John to turn around and say something along the lines of "is he really that much worse than you?"
John is changing and doing his best, but Arthur is human and emotional and John hurt him, and that's something that he does not simply forget.