The reason for this is that Mrs. Salmon should be afraid of what might happen to her. She is going to fear that the surviving Adams will come and kill her out of revenge.
We are told earlier in the story that she "went in fear" after the surprising verdict. The line that you mention at the end of the story simply confirms this. For all she knows, the Adams who was killed was the innocent man. If this is the case, the murderer is still alive and has reason to hate her. He looked straight at her over the dead body of his brother. What if he blames her for his brother's death and is going to come and get revenge on her?