With the recent shooting in Sandy hook this issue has come up again and I happened on this article.
1. In Pearl, Mississippi in 1997, 16-year-old Luke Woodham stabbed and bludgeoned to death his mother at home, then killed two students and injured seven at his high school. As he was leaving the school, he was stopped by Assistant Principal Joel Myrick, who had gone out to get a handgun from his car. I have seen sources that state that Woodham was on the way to Pearl Junior High School to continue shooting, though I couldn’t find any contemporaneous news articles that so state.
2. In Edinboro, Pennsylvania in 1996, 14-year-old Andrew Wurst shot and killed a teacher at a school dance, and shot and injured several other students. He had just left the dance hall, carrying his gun — possibly to attack more people, though the stories that I’ve seen are unclear — when he was confronted by the dance hall owner James Strand, who lived next door and kept a shotgun at home. It’s not clear whether Wurst was planning to kill others, would have gotten into a gun battle with the police, or would have otherwise killed more people had Strand not stopped him.
3. In Winnemucca, Nevada in 2008, Ernesto Villagomez killed two people and wounded two others in a bar filled with three hundred people. He was then shot and killed by a patron who was carrying a gun (and had a concealed carry license). It’s not clear whether Villagomez would have killed more people; the killings were apparently the result of a family feud, and I could see no information on whether Villagomez had more names on his list, nor could one tell whether he would have killed more people in trying to evade capture.
4. In Colorado Springs in 2007, Matthew Murray killed four people at a church. He was then shot several times by Jeanne Assam, a church member, volunteer security guard, and former police officer (she had been dismissed by a police department 10 years before, and to my knowledge hadn’t worked as a police officer since). Murray, knocked down and badly wounded, killed himself; it is again not clear whether he would have killed more people had he not been wounded, but my guess is that he would have.