Existentialism?

Existentialism. The start of my headache in English. From what I understood from the lesson is that people are only who they are through their actions. It doesn't matter what they say, actions are the only thing that describe a person. That's fundamentally what it is, right? While I may be perfectly fine with that, when this type of thinking is applied to everyone on the planet, it just doesn't make sense. Does existentialism mean that even actions from when I was a child still apply to me? Rambunctious, loud, mildly annoying. Ok. That may be true still, but that doesn't work for everyone. What if someone who committed a crime and wanted to turn over a new leaf? Are these people just known as criminals forever? How does existentialism even gain popularity with people? I can understand how scientific thinking could; it was practical. A person could go out, build something, test it, and be done. Something could be proved, disproved, or tested over and over. But all existentialism seems to be able to do is judge people. I'm this, you're that. Who cares? What's the purpose of labeling people?