Odysseus did not warn his crew about Scylla because he was afraid they would all hide under the decks and not fight the monster. He needed them to be brave, so they could escape the narrow passage between Scylla and Charybdis.
Circe warned Odysseus about the dangers of the two paths available to him after passing the Sirens. The first way was through the wandering rocks where King Aeolus, king of the winds, reigned. He was a volatile god and Circe admitted to Odysseus that no one had ever made it through that passage. She then described the alternative path through Scylla and Charybdis, noting that some of Odysseus' men would die with this route, but that most would survive.
Odysseus knew the sacrifice necessary to pass Scylla but thought it seemed to be the safer option compared to the wandering rocks where the entire crew was likely to be killed. Homer described the shouts of Odysseus' dying comrades who called out their leader's name as they perished. This haunted Odysseus, but he was still convinced that he made the right choice.
Odysseus put on his armor and fought Scylla, even though Circe explicitly warned him not to take part in the battle. This camaraderie with the sailors showed that Odysseus truly was thinking in the best interests of the group and, despite having to make a sacrifice of a few sailors to Scylla, he helped the remaining men as best he could.