If the main reason to kill off Robin (assuming he dies) is just to send Regina back over the edge or to make her suffer, it sends a terrible message that characters who are essentially good parents aren’t worth keeping around.
This. Also, that a person is only good for the sake of their significant other. I do have this bad feeling that they are going to use this to send Regina down the repetitive rabbit hole they’ve got Rumple stuck in. Sure, it’ll probably resolve itself by the end of the series, but it’s just horrible narrative strategy. But they’ve tended towards it with Rumple, and I’ve no reason to think they won’t use it with Regina. The drama would be whether she goes as “deep into evil” as she did over Daniel’s death — it would make a natural parallel. Anyway, I hate the idea of Robin dying — sure, he doesn’t really stand-out, but he’s also one of the least objectionable main characters on OUAT at the moment.