Now, in response to the question.............
YES......the Klan can and should be considered as a terrorist organization. Just like the New Black Panthers, any of the numerous street/neighborhood gangs across the country, motorcycle gangs such as Hell's Angels, Banditos and such, and several other organizations/groups.
These are all domestic terrorist groups who are actively involved in various criminal enterprises, and utilize fear and intimidation and physical violence to maintain their pursuits.