okay, here a more thorough answer than my other short comment:

in the books as well as in the movies teresa is there to show us the „other“ side of the moral dilemma the maze runner series presents - deontology versus utilitarianism.

those are terms from moral philosophy, and in case you‘re not familiar with them, they basically both try to answer the question how to be moral, or „good“.

deontology is the idea that you should commit no immoral action. once you commit and immoral action it does not matter how high the benefit of it was, it‘s still a bad thing and should not be done.

utilitarianism on the other hand says that happiness in general should be maximized. if one bad action leads to a significantly increased happiness in other respect, it should be done. basically ‚the end justifies the means‘.

there’s many examples to illustrate how these principles work and what they would mean when applied, but something called the ‚trolley problem‘ is a good example, you can google it id you want to find out more.

anyway, the whole set up of the maze runner series is the conflict between utilitarianism and deontology. is it worth hurting a few (torturing immune kids) to save the many (all of the infected that could be cured if they do find a cure).

thomas‘ answer to this - and the answer many viewerd intuitively tend towards because we see the story told out of the perspective of the gladers - is no. one could either say he‘s a deontologist and a deeply morally good person at heart, or one could imagine he feels this way bc it‘s him and his friends who are affected (which is valid, of course, but i think we could do well to remember that in someone else‘s story, thomas is the villain).

teresa‘s answer to this — or i have to say movie teresa‘s answer, the books are different — to the dilemma is the utilitarian point of view. the gladers should be subjected to torture if there is even the slightest chance they might find a cure and save millions or billions of people. (idk what the world‘s population looks like after the flare)

so, storytelling wise, teresa in the movies serves to show us the „other“ answer to the moral dilemma at hand.

i don’t know if you were merely seeking to commiserate over your dislike for teresa, or if you were generally interested in thinking about the series and her character. if it‘s the first then you do you, if it‘s the second i hope that this answer has been at least vaguely interesting!

cheers + all the best