According to his own testimony, Josephus was a Jewish general who surrendered to the Romans and became a prominent intellectual in Rome. Was he a traitor or a victim? Is he an accurate representative of rabbinic Judaism or a deviant? Surprisingly, the Talmud offers no opinion about him. R. Joseph B. Soloveitchik (Kinos Mesoras Ha-Rav, pp. 279-280) offers the following explanation of the Talmudic attitude toward Josephus:
The Jews under Josephus’ command fought heroically, but gradually the garrison of defenders was isolated and decimated by hunger. With no hope left, the warriors under Josephus’ command entered into a suicide pact rather than surrendering and falling into enemy hands. They drew lots and killed themselves one by one. Rather than committing suicide, however, Josephus surrendered to the Romans. He was taken prisoner and ultimately became a Roman citizen and was close with the Roman imperial family.
I would not say that Josephus lacked the courage to commit suicide. From a halakhic point of view, Josephus may not have been permitted to do so. The Midrash clearly says (Bereshit Rabba 34:13) that committing suicide is an act of murder. The case of King Saul causing himself to be killed is explained by some as an exception to this prohibition because Saul realized that the enemy would kill him imminently in any event. Therefore, if Josephus realized that he was not in imminent danger of being killed by the Romans, he would not have been halakhically permitted to kill himself. Nonetheless, one could argue that even though halakha may not have required Josephus to commit suicide, under halakha, one may commit suicide to avoid surrendering to the enemy. If so, such discretion would have been available to Josephus as well. In summary, I am not certain that Josephus committed an act of betrayal. He may have been acting in accordance with the prohibition against suicide.
It is interesting that Josephus is not mentioned at all in the Gemara, although Rashi does refer to him (Bava Batra 3b, s.v., hekhi). Our sages make no mention of the incident despite their extensive discussion of the events of the Second Temple period. It is hard to know what the attitude of our sages was, but I suspect that while they disapproved of what Josephus did, they felt that they did not have the right to condemn him. As a result, they ignored him. The Gemara would have mentioned him if it considered him a traitor.
UPDATE: I found the following alternate view by Louis Feldman, Judaism and Hellenism Reconsidered, p. 313:
Perhaps the silence of the Talmud about [Josephus] is due to the fact that he was an “outsider,” although Brull has attempted to find a hidden reference to him in a minor Talmudic tractate (Der. Er. Rab 5, Pirke Ben Azzai 3) which mentions a visit of several sages to a nameless (to be sure, pagan) philosopher in Rome seeking his intercession with the Emperor Domitian.”