Indeed, but the reward will be all the sweeter when he does. Remember, these are the only people that could possibly stand against an invasion; if Thresh breaks even a small number of them it might just be game over.

Again, what makes him more special than the other dark champions already in the league, who wield powers just as terrible as his? What does he bring that can actually break their spirits, when the champions have endured so much and still live normally and go about their lives as seen in the JoJ?

Thresh has too much competition to begin with. Whose to say Thresh himself won't suffer at the hands of others such as Malzahar, Veigar or Fiddlesticks, or outwitten by the cunning of LeBlanc or Swain?

He's perhaps more sadistic than most, but that doesn't grant him extra powers.

Aha, I see it. Thresh gets to torture the champions over and over again without permenantly killing them. Also, he can't permenantly die either. This ensures more eminent sucess in breaking the souls/ capturing them.

And then once he has broken the most resilient warriors in Runeterra, he can either A.) use this as a show of display to bring the world to his knees, therefore crushing every soul and using the souls to gain immortality, or B.) use the champions' combined life force from the core of his lantern to destroy the League or Legends and take rule of the world.

I figure Thresh would have originally joined the League without really knowing that he doesn't get to KEEP the souls he's harvested. Only after his first match or so does he realize what's going on and by then he can't back out. But there is another reason for him to stay there - it gives him an opportunity to become familiar with Runeterra's strongest warriors on the battlefield. By fighting with them and against them, he learns about their strengths and weaknesses and perhaps even their fears. I figure his participation in the League is like a psychopathic killer stalking his target beforehand: he learns all about them, down the details, can compile lists of everyone he's going to kill, and once he feels ready, can go off on a soul-harvesting spree outside of the League.

S'just my opinion of it though. I'm sure there're other equally valid opinions floating about.