I have two possible thesis statements for a paper on To Kill A Mockingbird: 1. "Encounters with injustice shape beliefs of morality by exposing its true wickedness." 2. "Justice is behavior or treatment that is morally right or fair. Encounters with injustice shape beliefs of morality by exposing its true wickedness. When we see something happen that we know and can feel just isn't right, we have to do something about it." Which one is the better thesis, and how can I improve it?

When it comes to a thesis statement, it's usually best to keep it to one or two sentences. Also, it should ideally be clear and precise. The second thesis you have is too wordy, but the first is too vague.
I would start by building upon your first thesis, specifically by giving it a greater connection to the novel you are discussing. Since the injustice you are talking about is being witnessed by the young Scout and Jem, I would include them in your thesis statement. This gives your thesis more clarity (how the themes of injustice and formation of morality present themselves in To Kill a Mockingbird) and makes it stronger so that you don't have to use more than one sentence to express it.
Your revised thesis might look something like this:

By showing the injustice done to Tom Robinson through the eyes of the young Scout and Jem, the novel shows how encounters with injustice can shape a young person's sense of morality.

Remember, a thesis does not need to lay out every argument—that's what your body paragraphs are for—but it does need to make it as clear as possible what you plan on saying.
Just make sure your final thesis accomplishes two things:
Clarity
Brevity