Because I know Oliver Queen is The Arrow.
Slade: Don't worry. I am not here to harm you.
Laurel: Go to hell.
Slade: All in good time, I'm sure. But before then, I've come to Starling City to see Oliver Queen suffer.
Laurel: Oliver? What?Why?
Slade: Because he's not the man you think he is.
Laurel: How would you know that?
Slade: Because I know Oliver Queen is The Arrow.