Slade is an ASIS Agent who was sent with his partner to rescue Yao Fei on an island. Their plane was shot down, and that is where Slade met Oliver Queen when he washed ashore after his boat accident. 

Slade, code name Deathstroke, begins to train Oliver on the island so that they might be able to escape. Oliver is captured by Slade's former partner and Slade kills his partner to save Oliver afterward they form a friendship. 

Slade turns up in Starling City to get revenge upon Oliver for the death of Shadow. A.R.G.U.S. has been calling him Deathstroke because of his worldwide killing spree.

Slade Wilson/Deathstroke Quotes

Sebastian Blood: You know who he is under that hood, don't you?
Slade: I know exactly who he is. He's my friend.

I will arrange for you receive another sample of my blood, and then you can begin to mass produce the serum again.When I ask you not to confront the vigilante, you will listen. Remember, your mask can be worn by another.

Slade

Arrow Quotes

Ra's al Ghul: You tasted death and you wanted more, but the truth is everyone and everything must come to an end. Even for one such as me.
Oliver: Kill me, but spare John Diggle's life. Let him go. I will beg for it.
Ra's al Ghul: You have shown tremendous strength, fortitude, power. No, Mr. Queen. I don't want to kill you. I want you to take my place. I want you to become the next Ra's al Ghul.

You know Oliver, somebody once told me that secrets have weight. The more you keep, the harder it is to keep moving.

Diggle