The event referred to in the question is known as the Pottawatomie massacre. This massacre saw John Brown and other abolitionists kill 5 settlers who were in favor of the institution of slavery existing in Kansas. This massacre happened shortly after the Kansas-Nebraska Act, in which citizens of Kansas had the ability to vote on whether or not they would have the institution of slavery. This vote caused voter fraud, fighting (like in Bleeding Kansas), and helped people to realize how the existence of slavery and its expansion was an issue that was tearing the country apart.
