Frankenstein monster is kinda like Trans folks or people

[Sheldon]

Well he claimed that we only have the subjective claim of the patient, which isn't true of course.

See my question above.

If a person behaves "like a man" but reports that they are a woman, what do we conclude?

I don't have enough objective information yet to form a sound belief about your hypothetical, but a qualified psychiatrist could test whether they are suffering from gender dysphoria or not, as was pointed out above. Again if you're interested in how psychiatry evaluate gender dysphoria, I can only suggest you go look into it.  

I certainly wouldn't liken them, or anyone else, to a "hideous murderous monster", even metaphorically. As the thread author did in a sweeping and generic way, and without demonstrating any objective evidence of course.