There was no way for Odysseus to get the men home without passing between the Scylla and Charybdis. Therefore, he is going to have to get his crew through that narrow passage. If he allows the ship to go down in the whirlpool, they all die. If he goes under Scylla, some will die but others will live.
What choice does Odysseus have? He has to take this course. If he had told the men they might not have been willing to try even though they really needed to. Odysseus is a leader. He has to do what is best for the majority of the crew even if they do not think his actions are right.
Therefore, Odysseus is doing the right thing. He is doing what he knows is necessary, even if that involves keeping his men in the dark.
I assume you are referring to Book 12 of this epic classic, when Circe gives Odysseus information about the various monsters that he will have to battle and endure in order to continue on his homeward journey. The particular monster that I think your question refers to is Scylla, a six-headed monster who has a great appetite for human flesh. Circe warns Odysseus not to fight this immortal creature, but Odysseus, perhaps emboldened by his success of making it through the sirens alive and hearing their song, determines to battle Scylla as well. Even though he puts on his armour and prepares his weapons, Scylla is too quick for him and snatches six men from his ship, and Odysseus is forced to do nothing as he hears their cries of agony as the ship passes on.
Therefore, personally, I think Odysseus was wrong to not tell his men about Scylla because of his worry that fear of this monster would force them to lose control of the ship. He showed great arrogance in not listening to the advice of Circe, and as a response, caused six of his men to die an agonising death. Telling them would have prepared them for the oncoming danger, and after all, they had faced many other dangers, so I am not convinced about the reasoning of Odysseus for not telling them. We see in this book that Odysseus wants to be a hero and to be seen as a hero, and foolishly risks the lives of his men for his own selfish motives.