I don't know where the "fundamentalist loner" terrorist stereotype comes from except from people who haven't been paying attention.  The only common denominator is they are all Muslims and they are not shunned by their Muslim communities - quite the contrary. From The Telegraph which carefully avoids the M-word.

British terrorists are just as likely to be married with children and have little religious background as they are to be fundamentalist loners, a leaked MI5 study has concluded.

Research from the security service found it was impossible to draw up a typical profile of a "British Terrorist" contradicting the perception that all are traditional religious fanatics or Islamic fundamentalists.

The classified report on radicalisation concluded there was no single pathway to violent extremism and no easy way to identify those who would become involved in terrorism in Britain.

Researchers concluded terrorists "are a diverse collection of individuals, fitting no single demographic profile, nor do they all follow a typical pathway to violent extremism".

The Home Office has refused to comment about the document.

The analysis was based on hundreds of case studies and found that unlike the common stereotype, most terrorists are unremarkable and live normal lives within their communities.

(...)

While most are male, women are often aware of their husband's and sons activities but do nothing to stop them.

Nor did anybody else in the community try to stop them funding, supporting, planning or carrying out violent jihad, also known as qital, also known as terror attacks. Other kinds of jihad are not even on the radar as yet.