His interlude is about kidnappers taking children, so mashu and the protag bring him in to use his abilities to track the kidnappers back to their base. I instantly love how he straight up offers them a chance to surrender, with the promise that no harm will come to them. He clearly means it. Hassan knows thats what the protagonist would want and acts accordingly. It really shows how he reads his master and their wants. All the awful stuff he did with zouken was just him following orders, and could easily be different under a better master.
The stuff with the kids being raised to be assassin’s, and how he gets pissed about it is super interesting? Like you’d think that’d be right up his alley (and he says as much himself) but instead he’s just like “mission accomplished. well, if you’ll excuse me master, im just gonna head off and beat the shit out of every kidnapper in rome.”
EDIT: i’ve been informed that my theorizing here was mistaken, and that chapter camelot reveals that cursed arm hassan was once a father. this would seem to explain his angry reaction to children being kidnapped.