Well, Regina did specifically say in the season 4 finale that her happy ending isn’t a man but being comfortable in her own skin. And then Robin’s death was heavily foreshadowed in 5×2 “The Price” when he nearly died. No price was actually paid then. So maybe a price needs to be paid now. I think Robin’s death has been planned since at least the end of season 4 when the writers realized how poorly Robin was received after the Zelena/Marian-Robin-Regina triangle. Sorry to say, but it looks like another father will bite the dust while Zelena will get to stay around and play mommy dearest.
"That’s how you know you’ve really got a home. When you leave it, there’s this feeling that you can’t shake. You just miss it." Neal Cassidy