Why Did Thea Kill Sara?

Some time later, it is revealed that Thea Queen killed Sara after being drugged by her father Malcolm Merlyn/Dark Archer as part of a plot to pit Oliver against Ra's al Ghul. Meanwhile, Laurel takes up Sara's mantle to fight crime as the Black Canary.

How did Oliver know Thea killed Sara?

During Wednesday night's Arrow episode "The Return," Thea finally discovers that she killed Sara Lance while under the influence of a special drug given to her by Malcolm Merlyn. ... Oliver assured her that she was not responsible for what happened to Sara. That the fault lies completely with Merlyn.

What drug was Thea on when she killed Sara?

Vitura was used on Thea Queen by Malcolm Merlyn in order to have her kill Sara. Malcolm then blackmailed Oliver to have to face the League of Assassins on her behalf and, were he to kill Ra's al Ghul, he would thereby clear Malcolm's blood debt.