what is rengar's role?

His role is ''Erasing everything that is not a tank from existence''.

jungle

Jungle is better, because he gets camped top vs good junglers - his only real escape us his ultimate. Most Rengars play aggro jumping from the bush while trying to kite the other toplaner with dmg for free and he is very easy target for ganks then, But many junglers let him to play like that for free and he can snowball with kills top.

Tearcollector

Why not both?

"top" and "jungle" are not roles, they are positions. Assassin, tank, ADC are roles. And Rengar falls into the Assassin or bruiser roles.

His role is to be as toxic as humanly possible.