Extremely large volcanic eruptions have happened in the past on Earth. For example the Deccan Traps
The Deccan Traps are a large igneous province of west-central India (17–24°N, 73–74°E). They are one of the largest volcanic features on Earth. They consist of multiple layers of solidified flood basalt that together are more than 2,000 m (6,600 ft) thick, cover an area of c. 500,000 km2 (200,000 sq mi), and have a volume of c. 1,000,000 km3 (200,000 cu mi). Originally, the Deccan Traps may have covered c. 1,500,000 km2 (600,000 sq mi), with a correspondingly larger original volume.
Some scientists believe that the formation of the traps might have contributed to a mass extinction.
The release of volcanic gases, particularly sulfur dioxide, during the formation of the traps may have contributed to climate change. Data points to an average drop in temperature of about 2 °C (3.6 °F) in this period.
Because of its magnitude, scientists have speculated that the gases released during the formation of the Deccan Traps played a major role in the Cretaceous–Paleogene (K–Pg) extinction event (also known as the Cretaceous–Tertiary or K–T extinction). It has been theorized that sudden cooling due to sulfurous volcanic gases released by the formation of the traps and toxic gas emissions may have contributed significantly to the K–Pg, as well as other, mass extinctions.
If you just pump it a bit it's plausible that such a volcanic event can terminate human existence, considering that as a species we are pretty high in the food chain.