Why was Darney rearrested after being released as a prisoner with the help of Lucie and Dr. Manette?

Charles Darnay was rearrested by the revolutionaries when Dr. Manette's old letters from the Bastille were discovered. The letters revealed that the doctor was thrown into the Bastille without a trial when he found out the St. Evermonde brothers were guilty of kidnapping and raping a peasant girl—then of killing her brother when the boy attempted to get revenge. Manette swears a curse against all who belong to the St. Evermonde house—and this includes his son-in-law, Darnay, since he adds "to the last of their race."
Of course, Manette wrote this decades ago, before he came to love Darnay as a son, but these facts do not matter to the revolutionaries. They rearrest Darnay and sentence him to death as vengeance.
This turn of events goes to show how the revolution became more about mindless bloodlust than justice or equality. Dickens portrays the mob as inconstant, praising people as their heroes one day and then guillotining them for treason the next.