Berlin's last sentence here is portentous: "I have no alibi for the night of the crime". The day of the murder, the Narrator is impatient; she imposes her solutions on Sara; she is neither open nor available to her friend. The Narrator says she had no alibi, but too late feels she also had no excuse for not being truly present to Sara.
People become entrenched when they feel their decisions or actions are being judged or criticized. Better to open a conversation about the relationship when troubling signs first occur. This intervention might have been more useful at the time of the unkind words, the slap. But Sara forgave Leon, perhaps over and over, and the friends did too because they wanted to believe in love.
She might have been more receptive if a seed of uncertainty about the relationship had been planted by someone who loved her when trouble first began.