I don’t think Sean wanted to leave though, because he said at one point he actually considered leaving because of all of the daily hate and lack of real storyline, but it was William Shatner who convinced him to stay onboard. I think Adam and Eddy just pulled the same stunt they did with MRJ to Sean. At the cons before the wrapping of this season, Sean always had ideas of where they could go with the character Robin Hood, unfortunately none of them fell through. Adam and Eddy gave us all the worst gift to give….hope.
So my guess as to why Adam and Eddy killed off Sean is because, much like you guys have stated above, they wanted to move WickedSisters forward and neither sister can have a true love, hence why both of theirs die in the same episode.
Budget cuts probably also had to do with the letting go because let’s all face it, OUAT doesn’t even make half of the money that Grey’s Anatomy does so they cannot afford to have that big of a cast.
I think the only question is why ABC kept quiet about letting Sean go when they clearly were not shy about firing two actresses (one the leading lady!) from Castle.
Trust me….if you go look at Adam’s timeline on twitter, you would understand. XD
"If you go as far as you can see...you will then see enough to go even further." - Finn Balor