Ok so I am going to explain why Oliver Queen is the one who killed Sara Lance.
Amanda Waller is hunting the League of Assassins
She wanted Oliver Queen to work for her in her crusade
Oliver Queen was held captive by her in Hong Kong where she injected him with a virus that attacks the nervous system
The virus does not affect the nervous system normally, but it needs to be fed with some ingredient to come into effect
The ingredient was present in Count's Vertigo that was injected into Oliver's system
The virus was activated and it programmed his brain to kill the members of the League of Assissins
He did not use his green arrows, but instead he used black arrows. Which is a part of the costume that he gets into when he is under the effect of that virus
Can't be Ras Al Ghul because he doesn't use arrows
Malcom has no motive at all to kill her
Arrow saw Oliver when he was under the influence of the new vertigo
Oliver wanted to choose Felicity over Sara
Arrow mentioned that he had to make bad choices in Hong Kong
Arrow didn't know what he was injected with, because if he had known, he would not be doing whatever he is already
Why did Amanda choose Oliver? She did not, she chose many others too (probably), or she chose him because she knew he had training from Slade Wilson.
She was surprised to see the person because Oliver was in a different kind of mask. A mask that Sara associated with someone else.
