Everyone's saying Cain deserves to die... but isn't he just gullible? In her first life, Aria was pretty awful to her service staff and did try to poison people, even if that idea was suggested to her by spies. From Cain's point of view, Aria had been acting like a villain for years and then tried to kill her own mother and sister.
Without all those reasons to distrust her and Aria acting like a saint, he naturally has a better opinion of her. Grabbing her arm is creepy, yeah, but in his view he's being protective because someone just tried to kill Aria. Also, he does have the self-awareness to know he's acting out of line questioning Aria's maids.
Does he need to back off? Yeah. Is he a doofus? Yeah. Does he deserve to die for what he's done right now? I really don't think so. Before he was playing into Mielle's hands, and now he's playing into Aria's.