quote hlelp from a Tale of Two Cities

[twisted__love]

"they are not easily purified when once stained red."
Charles Dickens

Can someone tell me what this quote means. I have to write an eassy and this is one of the quote. By the way it is from A Tale of Two Cities.

[D))N]

im not sure where its from in the book, i dont feel lik chekin but i think the red stands for blood, and that purified means that you forget about something.

thats pretty confusing here is my translatd vers When someone is killed, u cant forget them easily (if u know them).
So i think that someone is plotting to kill someone else to attack the heart of another person-- sort of lik spidr mn 1 when Peters best friends dad trys killing petes mom so that he can affect peter really bad.

[closedbyrequest]

No. The scene was from the storming of the Bastille. He was saying that once death had aproached the prison, it was not easy to purify it. Meaning that the prison was permanently tainted.