Hey guys! Thresh's Lore has the reason he joined the League in it!
 

And look, it didn't even ruin anything.

Wow, really? I, uh... If that's... no wait. I MUST THINK OF SOMETHING ELSE TO COMPLAIN ABOUT, ARGH



Oh I know! His name sounds too similar to 'trash'! What an example of horrible writing! Take that, Riot! Ahahahahaha!

Ohohohoohoohhoo!

Indeed my good fellow, indeed!

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?

So he comes to the one place on Valoran where people can't die.

Yeah, that's what I like about his lore. It actually does explain why he's in the League.

Well, it said he wants to break the spirits and sanity of Runeterra's strongest warriors, not exactly kill them permanently.

See... I have a theory just why Thresh, and more importantly, the Shadow Isle champions have been flocking to the League lately.

Looking at the world from the perspective of the Shadow Isles, every Champion, is a denied soul. They die, but are revived by the Summoner's magic. Wars of course claim many. But so far, nearly every conflict has been stifled by the League. Ionia's revolution and the Kalamandran war were both nipped in the bud. People are not dying as they usually would, including champions. All because of the Institute of War.

Take this as you will.

Indeed. But he does also have a penchant for souls as well. His lore states he seeks stronger ones.

Personally, I'm taking the same stance about him joining the League as Shaco, Cho, Koggles, Malz etc.

He didn't have much choice. But while he's in there, he's making the best of it; scouting out targets and discovering potential ways to break their minds.

It's pretty much confirmed canon that the Summoners really don't care too much about who or more accurately what they contain or what it's implications are. They're set and confitdent in their power, which hasn't broken yet.

And hey, I agree. It doesn't make too much sense if it's exactly as written, but at least it's something.

Sometimes the best way to break an enemy is from within. Understand them, play their games, learn their rules, and then exploit them.