Why did Liesel explode at Isla?

Germany invades the Soviet Union in late June, and Rosa loses her last customer: the mayor and his wife Ilsa, who have to cut back while they advise others to prepare for harder times ahead. On Liesel's last visit, Ilsa begs her to take The Whistler, which she does at first. But Liesel feels so angry that she returns and yells at Ilsa, attacking her for being wealthy and arrogant and telling her to face the fact that her son is dead. Liesel throws the book on the ground and sees Ilsa as having been beaten up by her words. Back at home, Liesel tells Rosa that she called the mayor's wife pathetic, and that is why Ilsa fired them. Rosa does not think Liesel is capable of insulting Ilsa for obsession over her dead son, and calmly accepts the news of having been fired.

The power of words to impel violence is explored in this part. Liesel's tirade against Ilsa is perhaps her cruellest moment since beating Ludwig Schmeikl. Liesel tells Ilsa, who has suffered for years over the death of her son, to get over it and pictures Ilsa's face as bloodied and battered from this verbal abuse. Liesel is genuinely angered about Ilsa firing Rosa, and does not regret what she said, though she later tells Hans that she is going to hell.