Why is Jane not inclined to condemn Mr. Darcy when she hears about his past conduct toward Mr. Wickham from Elizabeth? (Pride and Prejudice, chapter 16)

First of all, Jane is described as a slightly naive girl who doesn't believe that anyone can be an inherently bad and wicked person.

In this particular situation, she listens to the story that Elizabeth heard from Mr. Wickham, but can't blame either party. She thinks there is probably another side to the story. It's not that she doesn't believe Mr. Wickham's account and opinion; she just thinks that both of them, Mr. Wickham and Mr. Darcey, must have had problems of their own, that nobody else is familiar with.