i am not thinking that this idea is worse
Could someone please tell me why we don't use genitive here? (Negation)
"That" is more often omitted in English
Possibly, but it's safer to have some equivalent of "że" in the main translation. You may of course omit it, it will be accepted.
I dont think that this idea is the worst
"the worst" is "najgorszy", not "gorszy".