Hello! What if i control Necropotence and discard a card with madness ability, and i don't play that card. Will it be exiled from graveyard due Necropotence ability?

Nope, it won't be exiled. You're still discarding the card with madness, you just discarded it to exile instead of to the graveyard, so Necropotence will still trigger. However, when a card changes zones, it becomes a different object. Even if you choose to not cast the madness card when the trigger resolves and it goes to the graveyard, the card that's in the graveyard now is a different than the card the Necropotence trigger would be looking for, so the Necropotence trigger would do nothing when it resolves.

Nathan Long
Magic Rules NetRep