User blog comment:Reilock/Future in Runeterra's Lore: Thoughts/@comment-3058504-20130625070750

One possibility for kha'zix is that he joined ot league to eliminate rengar to take his essence and evolve to become stronger than before, since from what we have heard in both lores is that upon them both finding eachother their skills had been unmatched from the creatures they hunted.

A similar result could be deduced for Rengar aswell. With both of the champions seeing the league as a strong and effective way to meet eachother time and time again to prove who is the strongest.