i have finish the last scenario of core set. i actually resign but all the investigators were killed as the story claims. so now i must start over or i can make a new deck without xp and try the 3rd scenario with anothere investigator?

that last scenario is mean. technically everyone is dead if you resign.

i say you get a do-over (and now you know not to give up!) XD

otherwise, yes, you could start over with new decks either from the beginning or from that scenario (that's actually what i did).

Every scenario might be treated as part of the whole campaign or as a "standalone scenario". If you want to play just that scenario you can follow the deckbuilding rules for "standalone mode" (which let you build decks with some XP cards) and you'll also have to check the resolutions of the previous scenarios, because some choices might affect. Is the ghoul priest dead? How many cultists did you beat on the second scenario? Etc