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

Welcome home Kid.

Slade

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

Arrow Quotes

Your penicillin tea tastes like penicillin.

Oliver

Laurel: Helena, you don't have to do this.
Helena: Yes, I do. Because once you let the darkness inside, it never comes out.