The 1980 Mt. St. Helens eruption wasn't anywhere near as bad as it gets. It was a .24 cubic mile eruption. There was an eruption where Yellowstone is now that was 600 cubic miles*. Thats 2400 times bigger. Think the end of life as we know it.
Hopefully people won't be caught offguard like last time, so fewer people (if any) will die.