Why should someone be institutionalised if he feels he is Napoleon or Jesus Christ

Anti-psychiatry’s makes a startling claim that mental illness is just deviancy and goes furter to claim that illness of any kind, is deviancy,

The attribution of illness always proceeds from the computation of a gap between presented behaviour (or feeling) and some norm. At my fencing club, let us imagine, someone has taken a resentment towards
me. You don't understand why, you try to humour him, but no, the resentment seems set in concrete.
This behaviour carries on for months, one deems him to be unwell, for he is acting outside some norm. However it doe snot follow fron this very minor  straying from the norm that he is mentally
disordered.

And even if the behaviour was more extreme it  does not follow from the fact that someone can be justifiably described as mentally disordered that this automatically confers on the community the right of custody or intervention.

There is no a priori reason why someone convinced he is Jesus Christ or Napoleon should not be left at large