"doesnt know he's a psychotic cultist"
1. pretty sure most chaos cultists are psychos.
2. pretty sure we were told gotthard wittgenstein was a psycho, even worse than his family... who were all psychos. The least psychotic one was a talking cockroach.
Not all chaos cultists are psychos. However it is interesting that you think they all are, it makes a lot of sense.
I seem to remember a long and complex discussion based on the fact that Mortus was less convinced than everybody else of the psychoness of the von Wittgensteins. No doubt you have forgotten but you can go back and check if you want and while the von Wittgnsteins showed a certain lack of caring towards their subjects [which can hardly be considered abnormal], an interest in warpstone and a certain following for one of the chaos gods they appeared much more seductive than psychotic, but then I have forgotten that all cultists regardless of type are psychos.
Attacking people in a public place can not be considered a very good idea for somebody who is trying to remain unnoticed [but then all cultists are psychos and Gotthard wouldn't care that attacking somebody in a public place is going to bring the attention of everybody around onto him I'm sure. After all, all cultists are psychos.]
You have failed to explain why not letting a random stranger into the room you are in when you are fairly certain that you are being chased by a chaos cult and a friend of yours has just been nabbed by them is more 'moon logic' than letting a random stranger into your room in these conditions.
You have also failed to explain how a person is safer with a party that is openly, suddenly and without real reason hostile and whose leader is opening threatening as compared to not being with them.
However the real root of the problem is that, as this quote shows
doesn't open door to party member, runs away from them as it is safer to be on her own.
Finlay believes that the characters should follow the plot above everything else. Therefore from time to time he feels the need to engage in personal attacks. It is, afterall, what it is.