So any of you who saw Arrow Season 3 Episode 15 would have seen this big plot twist ending!
As we have yet to discover, no one knows why Malcolm Merlyn killed Sara. We know that he drugged Thea to do it. We also don't know why Sara returned to Starling; to search for Merlyn? - We don't know, neither do we know why Malcolm wanted her dead.
Ra's. Without surprise. Knew that Oliver didn't kill Sara. He doesn't seem amazed that this happens, where as Nyssa wont stop at anything to avenge her.
So as most of you know Thea served Malcolm to the League, to which he was taken after a fight with Laurel. Oliver goes to stop them, however they make way to Nanda Parbat with Merlyn. Oliver talks a few times with her after this saying it was a bad decision and that she wouldn't live with the guilt. He mentioned how he lives with the guilt of getting both their parents killed. He is not happy with her as she told Laurel that she killed Sara too.
As he is brought before Ra's you see his genuine fear as he begs for his life to be spared. Ra's assured him that he would be killed and that he would take no pleasure from doing it. To which Malcolm is dragged away kicking and screaming.
When Ray Palmer is building his Suit to which Felicity forces him to shower.They then kiss and sleep with each other, he suddenly wakes with the solution to the ATOM Suit. After this you see him Suit up and fly off past the window to the room Felicity is sleeping in. So where do you think ATOM will end up? What will he do? What will happen with Ray and Felicity?
Back at Nanda Parbat, Oliver and Diggle are filtering through Ra's' men. Upon discovering Merlin (who is beaten and struggling to breath), he mutters "Trap" to which the door slams shut and Ra's approaches the now trapped Oliver and Diggle. (To which you see in the picture below).
After this you see Diggle and Oliver sitting down discussing how they may die, where Diggle asks him to be his best man. Ollie says yes.
Oliver is taken and dragged out to meet Ra's. He begs Ra's to take his life and spare John's. Ra's says how Oliver is a strong person and an honorable man. He then asks him to be the next Ra's Al Ghul. - Yes that's right he wants Ollie as the new Demons Head!
Whilst this is happening Thea admits to Nyssa (who is in a cell in Oliver's [Arrow](series:720988) Cave), that it was her that killed Sara. Thea then opens the door and steps inside with Nyssa.