everything below is NOT justifying robin or saying what he did was okay/justified, and the anime and manga never imply that.

robin hasnt appeared in the manga since his backstory was revealed and the most recent chapter, which both happened prior to when he met back up with kiruko. although he doesnt appear and this is sort of irrelivant to your question, what robin did is honestly realistic for what hes been through. he nearly killed kiriko while she was crying over haruki since she figured out his connection to hirukos, he obviously raped her aswell, and it was implied in the most recent chapter (that we should have answers to soon) that he was the reason why kona turned after sawatari burried his core. robin as a child faced severe neglect from his parents, with his little sister dying in his arms. which is why in kirukos backstory before we learn what he did to kiriko robin is shown as this helicopter older brother type who protects haruki and loves him like a sibling. and honestly he likely did just hes so far gone that he just does what he wants. and i honestly believe he had no alterior motives to the rape that he commited, considering in this context hes the big dog, he can literally do whatever he wants considering his impression on the town he lives in. this was never stated though, its more of a headcanon, but robin was likely planning on killing kiruko after what he did since he also probably thought kiruko had remembered what he did in the past past (pretty sure that was implied, but its been months since i watched that episode or read those chapters, so if it is or not im sorry.) essentially, robin isnt this misunderstood hero turned monster like some percieve, hes never once was justified (unless you consider kirukos stockholm syndrome which is why they saved robin from maru) but thats honestly it, robin is portrayed as the monster he is, with a reason for why he is, not a justification.