If I may... A Not entirely complete theory...
It's obvious that the cure works as Roy isn't attacking everyone like a rabid minion in the promo, so Slade takes Felicity to kill her and destroy Oliver. In the confrontation, Oliver actually shoots her with an arrow and kills her himself... Hence the unthinkable as the title, cannot see Slade expecting that... Somehow (I've not quite worked this bit out yet) he has a vial of Miracuru and injects Felicity with it once Slade is dead, it works and revives her.... Then Oliver gives her the cure, to which she's back to normal (if the cure removes all the super strength and whatnot).
That would make for a hell of a confrontation.