Why do you think Odysseus chooses to sail toward Scylla rather than Charybdis?
This question is very clearly answered in book twelve of the Odyssey. Circe gives Odysseus advice about the perils he will face on his sea voyage as he returns home to Penelope. One of the major hazards she describes is a narrow strait he must traverse guarded by the two monsters Scylla and Charybdis. If Odysseus sails too close to Charybdis, he and his mariners will be sucked into a massive whirlpool, and they will all die. Obviously, that would be a very bad decision.
The other option is to sail on the side guarded by Scylla. Scylla is a monster with six heads. Each of the heads will grab and eat one sailor, meaning that steering toward Scylla will result in the deaths of six men rather than everyone. While this is not a good outcome, having six people die is better than losing his entire crew.
Odysseus refrains from telling the sailors about Scylla in advance in order to ensure that they do not become paralyzed by fear. Instead, they speed through the strait on the side guarded by Scylla.