When Judy realizes that her golf ball has struck Mr. Hedrick in his stomach, Judy offers a terse, insincere apology, then defends herself by reminding the men that she had called out "Fore!" (which in golf is the short way of saying "Look out: here comes my ball!"). Using this defense is Judy's way of disavowing responsibility for having hurt Mr. Hedrick. Then, without showing any care or concern for the man, she immediately and cheerfully continues playing her golf game, an abrupt behavior that the narrator calls "malicious." Throughout the incident, Judy's attitude is casual, as if Mr. Hedrick's being hit in the stomach with her ball is merely an inconvenience to her.
Here's that scene from the story:
"You hit me in the stomach!" declared Mr. Hedrick wildly.
"Did I?" The girl approached the group of men. "I'm sorry. I yelled 'Fore!'"
Her glance fell casually on each of the men--then scanned the fairway for her ball.
"Did I bounce into the rough?" It was impossible to determine whether this question was ingenuous or malicious. In a moment, however, she left no doubt, for as her partner came up over the hill she called cheerfully:
"Here I am! I'd have gone on the green except that I hit something."
Judy's behavior is important here because it reveals her true nature. She likes to act "ingenuous," all innocent and sweet, but she's truly "malicious." Specifically, she hurts people and finds it amusing.
This is the first time we're seeing Judy as a young adult. Recall that when we saw her as a child, her behavior on the golf course was equally brutal: she seemed ready to strike her nurse with a golf club. These little hints at violence within Judy foreshadow the damage she'll ultimately do to Dexter with her emotional manipulation. And the fact that we keep seeing Judy only on the golf course in the story (until she becomes involved with Dexter) is significant in terms of her character, too: this is a girl who plays games, and doesn't play fair.