I still think it's Thea.... If these flash backs like they been historically been doing been in connection with the present, then like Waller's training Ollie to become an assassin, the same way Merlyn is with Thea. The 1st assignment that Ollie had was to kill his best friend Tommy, it had to be someone who he knew. What makes you think that was the same with Thea? We know it had to be someone Sara knew but caught her completely off-guard. We know NOW that Sara was in Starling City to track down Merlyn, and Merlyn was INDEED in Starling at the time she was killed. So, why Merlyn was in Starling City then? This was five months LATER after he scooped up Thea. I believe that he and Thea flew back to Starling with Thea having her 1st assignment which was to kill Sara the same way Waller assigned Ollie to kill Tommy. I think that Malcolm manipulated the truth when he claimed "he did not kill Sara", meaning HE didn't do it personally. However, those ARE Merlyn's arrows struck to her chest. I think the whole Corto Maltese episode was a complete misdirection to assume both Malcolm and Thea were there the entire time.