The Black Death, caused by the bacterium Yersinia pestis, wiped out 30 to 50 percent of Europe's population between 1347 and 1351. But, this is just the most infamous of the little microbe's shenanigans. Y. pestis, which is one-millionth our size, has caused three major pandemics and continues killing people to this very day. The plague gets such a bad rap because it represents some of the greatest tragedies to ever befall the human race.
My grandfather used to keep all sorts of things in the trunk of his car: Fishing gear, duct tape, aluminum foil, a large chain, a defused WWII hand grenade. When we asked why he squirreled away such a random assortment of items, he would shrug and say, "Just in case."
That, in a nutshell, is why we should never destroy the smallpox virus. Just in case we need it someday.