In my opinion the word "stalker" has no place in this discussion, other than the OP using it. If you have even rudimentary understanding of attachment theory, it's easy to see what's going on. Unless there is some indication of malice, "stalker" is an inappropriate and pathologizing label to throw around.

As for therapists having a dangerous job… if they don't want to deal with this sort of behavior from clients, then they should not be engineering a relationship which by its very nature brings it out. I think it's ridiculous to lament the poor therapists in these scenarios, when clearly it is the clients who are suffering.

I agree that a client should bring the issue up if they wish to continue, but how many therapists are going to be able to handle it without hitting the panic button? Puts the client in a bind.