Robert Butler Jr. had transferred in November from a high school in Lincoln, about 50 miles southwest of Omaha.
In a rambling Facebook post filled with expletives, Butler warned Wednesday that people would hear about the “evil” things he did and said the school drove him to violence.
He wrote that the Omaha school was worse than his previous one, and that the new city had changed him. He apologized and said he wanted people to remember him for who he was before affecting “the lives of the families I ruined.”
The post ended “goodbye.”
Conner Gerner, a former classmate, said he remembered Butler as energetic, fun and outgoing. Gerner said Butler sometimes got in trouble for speaking out too much in class, but he did not seem angry.