A good episode.
I'm not sure I buy the "answer" to Sara's death. I can barely remember to what degree, if any, that Sara and Thea interacted when Sara (publicly) returned to Starling City. I would expect Sara to be a little more guarded and defensive if Thea mysteriously appeared on a rooftop dressed in a League of Assassins uniform. And I find it really difficult to believe that a drugged Thea with only three or so months of training would be able to shoot so many arrows into Sara without Sara being able to put up some sort of resistance or fight to defend herself.
The whole thing just seemed off to me. Unless it is more Merlyn trickery and he actually fired the arrows. But, then he surely would have need to be within Sara's sight range which should have prompted more defensive measures (on the basis that he wasn't already openly standing there with his video camera).
It's pretty bizarre how much faith Oliver has in Thea, but I guess that she is the only family that he has left, other than his, as yet, undiscovered son. I'm glad that Thea wasn't really a match for Arrow during their confrontation, but then it just made Thea's ability to murder Sara all the more mysterious and odd.
I'm hoping that they introduce the Lazarus Pit to resurrect Oliver. I'm not sure I would believe any tradition, non-comic book recovery methods since he seemed pretty done and dusted. I personally suspect that Ra's decided that Oliver would make a good soldier for the League and plans to use the Lazarus Pit to resurrect Oliver and then force Oliver to work for the League to pay off his debts (i.e. his resurrection and the fact that Ra's won't track down and kill Sara's true killer).
I'm looking forward to seeing how far they go with ATOM. And I'm hopeful that with Oliver's disappearance, they finally put Olicity to the side and pretend that it never happened. Felicity can move on with Ray. And if Oliver returns, then they can introduce a new love interest for him. Or bring back McKenna Hall who had a decent amount of chemistry with Oliver and with her job would be able to somewhat seamlessly be included in both Oliver and Arrow centric story elements.