What is the ruling regarding one who existed in this world and did not know Islam, and died upon that state? Is he blameworthy, or are the Muslims blameworthy? And does he die upon disbelief and thus enter Hellfire, or does he have a justification?

The one whom the call of the Messengers did not reach will be tested on the Day of Judgment. If he succeeds, he will enter Paradise, and if he fails, he will enter Hellfire. The deficiency might be shared between the one whom the call did not reach and the Muslims, or it might fall on one of the two parties.