No offense but (Bitch please) I main gnar, this adorable little ball of fluff is my life, I know what his lore is.
What I wrote there was basically a brief summary of how his lore went. Although, even then with Rengar's excellent narrating skills, he still wouldn't look like a sane person or admirable person after doing something like that, it would prove he was an ultimate hunter to be willing to do such a deed but I don't think Rengar really cared for attacking Gnar as post-fight Rengar just let it go with Gnar forgiving him. Hes focused on Kha'Zix as thats where his true ambition for thrill of the hunt came from. And just as one of my little assumptions, I assume Rengar would have a child-mentor relationship with Gnar as gnars lore has Rengar admiring the fact that Gnar is a hunter too.