Excellent question. Yes, God giving the command to kill Isaac is justification for doing so, except that God had also told Abram that Isaac would be an ancestor to many nations, which killing him would stop. Abram has to decide which statement is true. It is not based solely on what an individual believes to be right, which implies that anything goes as long as you believe it. Kierkegaard is saying that whatever the objective truth is, the individual has to make a decision to accept it or not, especially in situations where the truth is unclear, when the individual must resolve uncertainties and contradictions. He does also contend that at times we must depart from society’s ethical norms to obey a higher truth, which for him is God. But he never would agree with a “make up your own morals” idea.