Final answer:
Miss Havisham instructs Estella to break the hearts of men, teaching her to be an instrument of her vengeance against men after Miss Havisham's own heart was broken.
Step-by-step explanation:
When Miss Havisham tells Estella to "break their hearts and have no mercy," she is referring to men in general. In Great Expectations, Miss Havisham is a wealthy spinster who has raised Estella to be the instrument of her revenge against men, as Miss Havisham herself was jilted at the altar. Estella is taught to entice and ensnare men, breaking their hearts as Miss Havisham's had been broken, and she is instructed to do so without mercy, as an act of vengeance for Miss Havisham's own heartbreak.