Slytherin sealed the Chamber of Secrets so that none would be able to open it until his own true heir arrived at the school. The heir alone would be able to unseal the Chamber of Secrets, unleash the horror within, and use it to purge the school of all those who were unworthy to study magic.
"However, Lucius cared greatly for his wife and son and was one of the few Death Eaters who was capable of feeling genuine love. In the end, along with Narcissa, he demonstrated that his family was more important to him than serving Voldemort. Lucius’ love for Draco and Narcissa also allowed him to overcome his more cowardly side as he charged straight into the thick of battle during the siege on Hogwarts in an attempt to find Draco."