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 ? what he did to me, was outside the dojo and I still can't understand it. So I really don't know, if there is danger for me - surely not on the mat.
There are never enough facts in anonymous inquiries like this to enable anyone reading to give much in the way of a useful response, and additional facts tend to make the inquiry increasingly less anonymous.
That said, I would observe the following:
Aside from the fact that very few dojo operators are trained psychologists, psychiatrists, therapists, or certified social workers.....
Someone asked him to leave and someone let him back in.
Someone has implicitly taken responsibility for him.
You should talk with that someone directly. Information regarding your experience outside the dojo may change that individual's view of the situation. Or it may not.
But observing the way that individual deals with the information you provide and deals with your concerns will go a long way to letting you know whether this dojo is the right place for you.
If you don't feel you can talk with that individual, that tells you something important too.