Don't get too attached to Rengar, he is getting a "rework" on internal right now which largely just sounds like nerfs/forcing him to jungle.
That said, he is fun as hell to play, but his base stats and skills have all been nerfed so much that he isn't very good. Lots of lanes are really hard him for to survive, much less win, and he is completely dependent on getting ahead to make an impact lategame. You aren't going to be exploding any carries if you're 30cs/kills behind midlane etc. The biggest thing to learn with him in toplane (which is where you should be, his jungle is decidedly meh) is which lanes you want to level Q and which you want to level E. There are some champs you can just completely bully out of the lane or outright kill by maxing Q and going all in at 3 or 5, but the number is not very large (singed, darius, fiona, rumble all explode pretty well). Otherwise you mostly wind up leveling E for harass vs the rest, or if you're really getting crushed W just to waveclear better and heal more often (lower CD, more 5stacks). Do not overlook an early kindlegem either, especially if you're ahead. Brutalizer is a fine first item if your lane is going well, but if you're losing the better path is often kindle gem/vamp scepter early.
And don't built sword of the divine, it's terrible win-more bullshit that you don't need.