At first, Saul's words and deeds were more in line with God's will, but later he failed. First he sacrificed privately against God's will, and then he disobeyed God's command. Samuel criticized him, and he defended himself, so God left him.
People are also unreliable, and Saul is the representative of people. People's obedience will not last long, and people's love will retreat, so don't listen to people's vows. Human life is flesh and blood and extremely unstable, so people's obedience and love can only last for a while. It can be seen that human life needs to be updated. Jesus came to the world to renew human life and believe that he can get a new life.