Josephus Flavius betrayed us and joined the Roman Nazis. How is he NOT a Kapo?
Regarding his work, Josephus' work is considered to be a Sefer Hitzoni (ספר חיצוני) and much of what he writes are obvious lies. There might be an issur for reading his work, but I'm not sure. So even if I don't know, I don't takes risks. That's what Jews do. And what you say about his work can be said about the New Testicle.
Josephus was probably a traitor kapo. We don't know for a fact that he helped the Romans by intel and advice to fight the Jews after he had been captured because we only have his version. But his account of the war is the most detailed and much can be learned from it.
His works are historical and not spiritual so I don't think there is any cherem to apply to them. One thing though, is that the Christians in the middle ages forged and rewrote some passages in his books concerning Jesus (Flavian Testimony) I think on "Antiquities" where he is supposedly calls Jesus the messiah etc (the so called "Flavian Testimony"). Josephus was actually a pharisee.