I see that alot of the champs lore dont have a reason for joining the league look at Kha zix, Rengar and now zed. They should have like a badass lore saying why they joined the league how they got there and such. What does the community think ?

For Khazix and Rengar, I assume they joined to get more chances to fight eachother, and fight worthy prey.

But yeah, I've noticed this too. Usually, there are passable explanations.

Take Elise for example... now everyone knows she is a spider shapeshifting monster, why would she join the League?

Death is a promotion in Noxus, some will willing embrace death to become a summoner of Noxus after death. Afterall, when you are consumed by the Vilemaw, only your corporeal form is forfeit and you are now a part of the Shadow Isles.

Rengar joined the League to prove that he is the ultimate hunter, he wanted to test his skills with the best fighters in Runeterra. Kha'zix joined for essentially the same reason. As for Zed, I would imagine he joined since Shen is in the League as well as to show off the dominance of his ninja clan. Elise joined for the Shadow Isles, now that it seems they want representation in the League.