Apologies for the unavailable link.
The relevant passage is here:
If you know from your own personal awakened experiences in the internal worlds that you are a bodhisattva, you have my sympathies. Truly, to be so is a great disgrace.
But, many of the people who think they are fallen bodhisattvas are not. The common claim of being a fallen bodhisattva is an infection throughout spiritual groups that only makes a mess of peoples lives. It is strange that people want to believe they are fallen bodhisattvas, when we know from the teachings that fallen bodhisattvas are worse than demons and should never be trusted! This belief that people love so much is 99% of the time just pride encouraged by misguided spiritual groups and leaders who, by depicting themselves as the worst, want to be envied and respected as the best.
My advice is to be very doubtful of your idea that you are a fallen bodhisattva. Pray you are not a fallen bodhisattva. Prefer not to be. And IF you are, do not discuss it with ANYONE, EVER.
Anyway, whether you are or are not is totally meaningless and useless. Whatever one was, you are what you are now, and the path remains the same.
And, as to why a fallen bodhisattva is so dangerous:
For the same reason that atomic energy is more dangerous than a match. A bodhisattva can facilitate more energy, whether for good or evil. This means they tend to be magnetic, influential, and capable of acts beyond the abilities of a common person: again, whether for good or evil.
As to how one would know if one is a fallen bodhisattva, only by awakened experiences in the internal worlds. If anyone ever tries to tell you that you are one, avoid them; they will only bring you grief.