I believe the answer is: It suggests that she did not care about his death.
Pay attention to this line
And then she—laughed. I guess you would call it a laugh.
Laughing after the death of loved ones is very unusual, but it is not impossible. From her action, most readers would most likely come up with two possible explanations:
The first one is that she is mentally shaken to the point where she laugh like a disturbed person. The second one is that she laugh because she is the one that murdered him and she is satisfied with the outcome.