I mean bindusar asked him twice, asked him to tell him directly, why wait for proofs??
Bindusar knows chanakya will never lie, so if chanakya had opened his mouth right time, Justin would have been saved and Helena would have been sentenced to death but then how can story continue if main villain is dead.
But this whole thing of chanakya knowing the truth and still quite is making him look really responsible for all wrong doings.
Now if chanakya opens his mouth against Helena, nobody will believe him becoz she killed her own son.