Why are some tornadoes more upsetting than others?

There are a number of factors including the strength, size, path length, forward speed, and location of the tornado. These play into how bad the damage is and how many people are killed or injured.

Tornadoes vary greatly in strength. Most cause only minor to moderate damage (EF0 and EF1) but a few are strong enough to completely obliterate houses (EF4 and EF5). These tornadoes are often very large and long lives as well, so not only is damage very severe, but it is also often widespread. These tornadoes are the ones that most often kill large numbers of people whereas weak tornadoes rarely kill. A large tornado with a long damage path will affect a larger area and thus cause more damage. If a tornado is fast moving there will be less time to warn people, making it more likely that people will die.

The location is also very important. Many very strong tornadoes have torn across open farmland, causing relative little damage. However, when a violent tornado hits a city, as was the case of Joplin, massive destruction will occur and multiple fatalities are almost guaranteed.