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?
Thresh now haunts the land, leaving an aftermath of horror and despair. However, there is a devious purpose behind his dark machinations, and the meager spirits of average men are insufficient. He seeks stronger souls. Only when he has broken the wills of Valoran's most resilient warriors will he finally have what he needs.
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?