Even at the the far edge of the mud and ash that came from the Pompeii volcano's explosion, the heat was sufficient to instantly kill everyone, even those inside their homes.
And that is how the people at Pompeii, whose remains were found trapped and partly preserved within ghostly body-shaped tombs within that pyroclastic flow, died. They did not suffocate. They did not get blown apart by force. They did not die of gas poisoning. They simply cooked. Instantly.