Why is a pyroclastic flow the deadliest thing a volcano makes?

A pyroclastic flow is a fast-moving current of hot gas and volcanic matter that hugs the ground and races away from a volcano. It is the deadliest of all volcanic hazards, and the reason is a combination of speed and heat. Pyroclastic flows average about 100 km/h and can reach 700 km/h, and the material inside runs from 200 to 700 degrees Celsius. The kinetic energy flattens trees and buildings, while the hot gases and speed quickly incinerate living organisms or turn them into carbonized fossils.

The most famous victim is Saint-Pierre on the island of Martinique. On the morning of 8 May 1902 a ground-hugging cloud of glowing particles and searing gas, a pyroclastic surge, rushed down Mount Pelée at hurricane speed. It reached Saint-Pierre at 8:02. Almost everyone in the city, about 28,000 people, died, most burned or buried by falling masonry. The government had deemed the town safe because of hills and valleys between it and the volcano, despite signs of an impending eruption. One of the very few survivors in the city was Ludger Sylbaris, a prisoner locked in a windowless jail cell, found afterwards by rescue workers.

The same thing buried Pompeii and Herculaneum in AD 79, where Vesuvius's surges engulfed the cities with many lives lost.

So why does the cloud travel along the ground instead of rising? When an eruption jet cannot heat the air around it enough, convection cannot carry the plume upwards, and it falls and flows down the flanks of the volcano. Heavy material hugs the ground and the lighter ash and gas keep flowing across the ground and air, and a surge's low density lets it flow over hills, so even a hill between you and the volcano may not help.