High Arthur might have killed hundreds if not thousands of people, turned wives into widows and children into orphans, but he also saved hundreds of fish by freeing them after catching them, and politely greeted anyone he met, even if he robbed or killed them later. That's why Arthur is better than us. He's a good man.
People always say that he’s not Arthur just does not want to think about the brutality of his poor death so he try to think of a positive instead of the horrible thing he just witnessed.
I think it was more based on the fact that Arthur feels guilty about Kieran's death with High Honour than he does with Low Honour. In low honour, he just says poor kid died and they tried to kill us all. Even the sigh in his voice isn't of sympathy, it's more of pity or "welp shit. this happened. another one bites the dust" kind of an attitude. In high honour, he actually feels guilty of the fact that he couldn't return the favour back to Kieran when he saved his life back at Six Point Cabin. Plus, he's emphasizing more on emotions in high honour about how he's feeling and what love's done to Sadie rather than monotonously narrating what happened in the journal. Even his voice sounds like he's actually sad when narrating.
If you actually look in his journal, High Honour or Low Honour, he writes both of which he says in the respective scenes. Same for other moments like Sean's death. The game just puts more emphasis on one of the particular expressions based on what Arthur would be wondering about more depending on his honour.
