The Subaru being her "reason to die" line had quite an impact on me when I first saw it. Compared to the usual romantic "reason to live" line it sounded horrible at first, but thinking about it more deeply I think it's far more indicative of a purer love or selfless love vs obsession or dependence. "A reason to live" line could be utilized in the latter situations. Subaru being her reason to die shows the depths of her feelings in that she's willing to sacrifice everything for him. Its not just empty words either given her actions in Ep. 15. I also think it means more because there's a very high chance she's aware of Subaru's feelings for Emilia. It seems highly probable from her commenting on being happy that she got him to herself in the capital to everyone else seeming to realize (I mean come on Crusch picked up on it within a day) it save Emilia herself. (I'd normally chalk this up to denseness, but I think she's just been so despised being similar to the witch that she doesn't even think it possible for someone to love her)
One theory I like myself is Rem becoming the witch in the future + her hopping back in time to ensure Subaru comes to the world/save him/reunite with him if he died originally without Return by Death. If she starts becoming envious of Emilia and mind breaks like everyone else and their mother in this series so far, I wouldn't put it past the author to have her desperately attempt to appeal to Subaru including, by say, using magic to take on an appearance similar to Emilia. Return by Death could also stem from her intense desire to safeguard Subaru's life too, especially if he's offed in a bad end and she snaps because of it. The only thing questionable is why would the cultists would attack her village, but they're all bat shit crazy anyway, and you try 100% controlling a bunch of nut jobs. Plus she might be totally mad by then too.
On a more lighter note, is it just me or has Rem only gone into her oni mode so far because of Subaru? Queue "horny" jokes.
