Originally Posted by Melancholy Exile
So ... why did he join the League?
I understand the element regarding his interest in strong souls, but what does he gain from submitting to the League and why on Earth would he think to do so anyway? Why does he need to take to the Fields of Justice to break the spirits of its members?
Beyond that: Why would the Institute want to admit him to the League if they thought such a thing was possible?
So ... why not haunt them away from the League and the supervision of Valoran's pre-eminent mages?
Why would a ghost see fit to submit to someone else's rules and regulations, rather than simply taking souls as he pleased on his own terms?
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.