I think they could have explored the relationship for atleast a few more eps before they killed her. But in that case, her death wouldn't have been as tragic, in case it didn't live up to the expectations of the fans. Tough call.
Otherwise, i think it was written really well. Well, that's what I say no, after having dealt with that they teased us since the beginning of season 3, making us think they might get together and then taking Fred from us. I had mayor issues with that at first, but now i'm more objective and can say it was brilliant.