He is a danger for people and himself, has been banned from the dojo for 6 months (for threatening somebody) and came back recently. But aikido is for educating people who have problems with violence, isn't it ?
Yes and no. IMO, A dojo is not responsible for rehabilitating people nor is a place to "experiment" with people. As Mr. Little said, most are not run by Psychiatric or Medical Doctors that specialize in this field. Even if you have such an individual, it is not a clinic, nor is it a place for treatment.
It is a place where people should be able to come and train safely without concern for their safety or health. If a member of the dojo poses a risk to anyone, and it is identified, then it must be properly dealt with and mitigated.
Yes, it is a place for sane and stable people to come together to better understand violence. it may even help those with bad temperment, and those that are prone to emotional and physical outburst. However, if they ever are considered to pose a danger to the health and welfare of others, then they should not practice there. There is a clear line between having feelings and issues, and acting on them uncontrollably.
If a professional thinks that aikido is good therapy, then he/she should start his/her own "support/therapy" group that is composed of people that williingly place themselves in that environment as "assistants" to the professional, or as "patients". The environment should be controlled as a clinc or support group.
A normal dojo is not such a place, IMO.