I think the bolded text sounds OK.
(I could offer possible stylistic improvements. I prefer "fanatical" as an adjective here, but "fanatic" is a valid adjective as well, and people refer to "fanatic Nazis", even if not as often as they refer to "fanatical Nazis"; I would also quite like to insert "just" before "a few", or else change "in" to "within", for reasons that I can't quite explain and which may well be wholly subjective. But I believe the sentence is correct as it is, and not especially unnatural.)
You can "become" + an adjective, but you can also "become" + a noun. You can become a Christian, become a Muslim, become a Communist, become a Republican, become a Nazi, and so on.
Among Cambridge's examples are:
Margaret Thatcher became the UK's first woman prime minister in 1979.
He has just become a father.
Of the four million people who have become vegetarians in Britain, nearly two-thirds are women.
When did you become a US citizen?