I’m going to be talking about the books later so is a deep dive on her betrayal in the movies.
For one, people say she should have known that WCKD wouldn't not hurt anyone. While this may seem like common sense to most, we have to remember that Teresa grew up with them. I'm her memories, they do good. Because of their goal and the way they took her in when she had nobody, it’s reasonable for her to trust them and have a bias.
Next, in The Death Cure we find out they have trackers in their neck. That means that WCKD was bound to find them sooner or later. Her telling them only sped up that process. There was no actually getting out of it.
People say that she is the entire reason Newt became infected. This is a flat out lie. While they did go to the city because of Minho being captured she did not give him the Flare. There are two possible places he did. The first one is when their car got flipped over. In that scene Newt does end up holding his arm. This is from one of two things. Either he just hurt his arm in some way or a Crank bit/scratched him. The other possibility is the movies followed the books when it comes to the virus being airborne. Either way, Teresa did not plan on them coming to get Minho nor did she plan on this happening. She was looking for a cure that would have saved him. Some people say it was due to her long speech. However, that long of a speech could have actually saved him. When we see her giving the speech that is all she is doing. However, someone else turned on the lights, and light makes Cranks violent. That is most likely why Newt woke up and start attacking Thomas.
Now she did end up finding the cure. She could have saved everyone, but there was a war going on in the city which was already killing everyone. This was not something she could have predicted. If that hadn't been happening she very well could have saved the world.
Another huge factor to her not being able to is Janson. Janson has only self interest. He does not care about people who can't give him something he wants. That's why even when there was the cure he wanted it all instead of making more for any survivors of the war who were infected. When Teresa realized this she turned against them completely. She decided to save Thomas instead of something that she had dedicated almost her entire life to and had lost people over. She cared more about someone who hadn't tried to understand where she was coming from instead of what meant so much. She saved him and dragged him up through a burning building. Not only that, she apologizes for her actions and says she tried, showing that she blames herself despite so much of it being out of her control.
Her death was a suicide. That is what makes people who say they laughed when she died do more than get under my skin. They also say Newt would have been happy, but I disagree. Newt was selfless in a different way, and I truly believe he would be devastated to know she had committed suicide.
How do we know this is a suicide? Well, she has people reaching for. She has the choice, but she saved Thomas. Then, she doesn't see the point in going on. She feels so much guilt that she dies a horrific death of burns and falling to the concrete from a tall building. Not only would it be extremely painful, it wouldn't feel so quick even if it may have seemed that way. She would literally die in flames. She was not standing there because she was frozen in fear. She stood there because she thought she didn't deserve to live. The way she was able to stand after part of the building fell and get up with her head held high proves it. That is absolutely awful and unfair of her to believe and feel.
Lastly, she was a teenager. The estimated ages of everyone in the movies from what I’ve seen is sixteen to nineteen. No matter where she fell her frontal cortex was not fully developed which can cause more impulsive or irrational decisions. While it was clearly a very difficult but stern one and she stood by it, my point is that she had so long left. She could have learned so much about herself and grown even more, until she was someone that could be more healed and happier.
Teresa Agnes took her life as a teenager because she felt guilt for so many things out of her control.
Teresa Agnes is innocent.
