Because it's totally right when you kill an innocent man and his wife and child, all because that man happens to be the son of the man whose brother raped your sister.
That makes sense, sure.
Revenge in general is not a pleasant thing. When it is carried out to such extremes as Mrs. Defarge in A Tale of Two Cities carries it out, not only is it unpleasant; it is destructive, horrible, unfair, more wrong than usual, and the sort of thing that makes everyone hate you.
Besides that, one of the qualities of Defarge in this book is the quality of not stopping. That is to say- she no longer sees any difference between people. There is no difference between the prisoner and the free man; captured, free; guilty, innocent.
That is why you shouldn't be like Defarge.
(does this post make any sense so far?)
Defarge had a horrible back story. I will admit. It sounds traumatizing and horrible and scarring for life. However, I do not believe that her backstory gives her the right to accuse and bring to death the innocent characters in A Tale of Two Cities.
As dramatic as that sounds...
It's altogether possible in today's day and age. Think about it. Someone does something wrong to you. Let's say it's a parent of someone you know. Because you can't take revenge on this parent, you decide to be cruel and bully-ish to her children.
Do they deserve this?
Although you aren't accusing them of treason and having them executed in a literal sense, think about what you're doing to their brains. You're basically making their life a living hell.
Because you happen to be mad at their mom.
Or, to set another example...
Let's say you hate a couple of people your own age. And you try to take revenge on them. However, there is also someone else (who you'd ordinarily not mind at all) who is friends with these people. So, you begin to shun her and purposely not invite her to things (and then say "shhh... don't tell her about the party!" knowing it'll get out).
Does she deserve it?
All that's happening here (and with Defarge) is that she's taking all her anger and everything... And transferring it onto someone else. She's taking all the things that she'd like to do to the original person (Darnay's dad and uncle) and transferring it onto innocent people who are vaguely connected to them (Darnay, Lucie, little Lucie). Why? Because she happens to be mad.
So, I'm sort of begging you, in a round about way... Do not be like Defarge. Be kind. Forgive the people. Try to ignore the people who tick you off. And... Whatever you do... Do not try to take revenge. Because revenge is destructive (as illustrated by Defarge dying) and it will make people hate you.
Have a nice day!