Bystanders at party watch 5 stabbed.

http://www.nydailynews.com/news/crime/calgary-stabbing-victims-identified-students-article-1.1758143

This article describes a house party in Calgary that was being held by college students to celebrate the last day of classes. However, one student violently attacked and stabbed 5 others in front of everyone at the party. The attacks were allegedly unprovoked, and my guess is that the helping behavior broke down in-between steps 2 and 3. Everyone at the party clearly interpreted the emergency, but nobody stepped up to disarm the attacker. I believe this was because they weighed the benefit of cost vs. rewards, and that all of the bystanders were too worried for their personal safety to intervene. The killer was invited to the party, seemingly another friend of the students, so it is probable that the majority of guests knew the kid and decided not to help in the face of a large kitchen knife. He was the son of an ex calgary police officer, and the reason for the attacks has not yet been determined.