I think these sentences in the excerpt from F. Scott Fitzgerald's Winter Dreams reflect Dexter's heartlessness:
"It was strange that neither when it was over nor a long time afterward did he regret that night".
"And gave serious hurt to Irene Scheerer and to Irene's parents, who had befriended him":
"There was nothing sufficiently pictorial about Irene's grief to stamp itself on his mind".
In these sentences, the reader can understand that Dexter is a selfish, uncompassionate person that has no empathy or even kindness for others.