I used to be of the view that the status or validity of a guru or master cannot be judged by their the nature of followers. In other words, a guru or master cannot be blamed for the idiocy of their followers. It is not Jesus' fault that some Christians became missionaries and destroyed the cultures of those people conquered by Western colonialism, while others try to impose their paternalistic moral standards on everyone else. It is not Mohammad's fault that women are oppressed under Islam (no this isn't just Western propaganda, I have heard from at least two very different and totally independent sources regarding this) and that Islamofascists have no regard for human life. Or that Marx or Lenin were to blame for the Stalinist purges and the Gulag.
I no longer believe this. While one cannot really blame Jesus, Mohammad, Marx, or anyone else for crimes committed in their name long after they were gone, in the case of contemporary gurus and their organisations, the situation is very different. What got me thinking this way has been the association of slanderous devotees full of extreme shadow projecting with the highly controversial sexual abusive guru Sai Baba. Sai Baba (who I used to consider a genuine avatar for some quarter of a century, before the allegations became public) and his movement constitute the extreme case I know of where an abusive guru is defended by several highly antagonistic, hateful, and outright slanderous devotees who will do anything they can to insult, defame, and blacken the names of people who either point out allegations against their guru, or refuse to accept their guru's claims (this includes cunning manipulation of Google and other search engines). I also noticed exactly the same tactic of slander and ad hominem attack used by Scientologists against anti-cult activist Rick Ross; see Rick Ross Responds to his Critics.
On the other hand, I have found the devotees of Adi Da to be perfectly decent and civilised people, despite their guru's questionable behaviour. Maybe not all devotees are like that; I don't know, but those who have contacted me (taking issue with what I have said on my website) or who I have discussed things with (see e.g. this conversation on Wikipedia Talk - The Intermediate Zone are perfectly nice people.
Similarily, although I have been very critical of the Ameerican philosopher Ken Wilber (who does not call himself a guru and is not an abusive individual, but has a guru-like following), I have found, with only one exception and one rather weak attempt at dismissal of one of my essays, his followers to act with the utmost decency towards me. This isn't to say that they don't argue against my criticisms, but rather they never resort to deliberate and pre-meditated ad hominem attacks that the afore-mentioned Sai Baba devotees do (I am absolutely not saying all Sai Baba devotees act like this; only certain ones, perhaps a tony, if exceedingly vocal, minority). And even that exception afterwards apologised to me, something that the slanderous devotees of Sai Baba have never ever once been known to do.
But just my experience of Wilberians. Frank Visser, who hosts a Wilber criticism website and blog has certainly come in for ad hominem attack by Wilber followers. But this is almost certainly because Wilber himself ridiculed Frank in his own blog, so they would be following the lead of the Master.
I myself have never, thankfully, been threatened with physical violence from disaffected devotees. This is because most guru devotees are a pretty peaceful lot. But still there are ugly instances where this does exist; for example in the case of Mataji Nirmala Devi's Sahaja Yoga, and Muktananda's Siddha Yoga, where guns, threats of violence as well as sexual abuse are well documented. This is, incidentally, the same Swami Muktananda who I have always felt such a lovely energy from! The Intermediate Zone is beguiling and paradoxical indeed!
So it seems to be that in some cases a guru can be judged by the bahaviour of their followers, in other cases not. A lot also depends on the nature of the guru, obviously. But a lot also depends on the devotee
With genuine gurus, there is no problem here, because the exemplar is always of a very high level, and a devotee can surrender with sincerity and follow a guru with true bhakti, using that guru as a form of the Divine. This does not stop the inevitable cases of people around the guru going crazy, due to the force odf their presence and the Intermediate Zone in general. Even such flawless gurus as Ramana Maharshi and Sri Aurobindo and the Mother can have devotees who have a weakness in the adhara (the psycho-physical being), and who start to believe that they have a unique cosmic role, are also themselves avatars, and so on.
But with false gurus, the situation is very different, because often a devotee will have a much higher moral and spiritual consciousness than the guru himself. Then, after many years of service and devotion, and faced with evidence of abusive behaviour (and in some cases even being on the receiving end of this), the devotee is faced with a difficult choice. Either leave, with the knowledge that everything one has gone through, all those years of service and sacrifice, and in some cases all that experience of abuse, have been for absolutely nothing, or go into denial when faced with the allegations, and as a result resort to shadow projection and taking on the qualities of antagonistic and even slanderous behaviour and paranoia as a way of rebutting the overwhelming evidence of the critics.
While the basic lines of evidence seem clear, more research needs to be done, regarding the nature of the morality of false gurus and followers, and whether sincere devotees take on the negative traits of abusive gurus, or are sincere and spiritual even in spite of the imperfection of their guru.