Over the course of the show's first two seasons, we got to watch as Slade Wilson fell from grace as Oliver Queen's friend and mentor when the two were stranded on Lian Yu to become one of his most dangerous foes to date. Driven mad by Mirakuru, which created within him a blind hatred for Oliver over the death of Shado, Slade inevitably resurfaced in the present day to tear apart Oliver's world.
Thanks to his actions, Oliver lost his company and his own mother, creating a massive shift in his life that has remained somewhat unparalleled by the other major villains Team Arrow has gone up against since. Fortunately, though, thanks to the passage of time and the Mirakuru having finally worn off, Slade has since seen the error of his ways, which has earned him his freedom from his imprisonment on Lian Yu, allowed for the rebuilding of a positive, if rocky, relationship with Oliver, and put him on a path of redemption.
Thanks to his personal connection to Oliver and the strength of Manu Bennett's performance, Deathstroke has remained one of the show's most lauded and popular villains after all these years, having raised the bar that Merlyn set that all his antagonistic successors had to try and clear, and his new lease on life has given him a whole new world of potential for where Arrow - or even the Arrowverse at large - can take him going forward.