So far we have seen that Sarah arrived on the boat, was brutalized, and then Ivo's intention was to use her in some kind of experiment.
When she next appears, Ollie is on the freighter, she is acting without affect, kicks him around and is holding a knife.
The connotation is that she is going to give him some more of those scars that everyone asks about.
It appears that in the time frame between the Queen's Gambit sinking and her kicking Ollie on the freighter, she has been tortured to the point of becoming a torturer.
This may not end up being true, but all points made so far lead to this conclusion, and there is no evidence of another situation.
This is still speculation based on a lot of assumption.
I could just as easily say the lack of emotion was her poker face. The knife is to free Ollie's hands. And the very next words out of her mouth are going to be, "Oliver, what are you doing here?" Chances of that are slim since the show rarely takes the obvious maudlin route, but it's still as good any other guess.
There's nothing wrong with speculating. The thing is, calling her an "evil assassin" is a big leap from suggesting she might be acting irrationally and instinctively under extreme physical and psychological duress.