How do you know it's right? You can't run the same experiment of life twice so you only get one shot

that's not how morality works... morality is about doing what you think is right instead of excusing yourself not doing it. If you think a bad thing is going to happen that you have agency to prevent, that's on you. Whether you have the data from repeated events or not doesn't matter. (Of course, your confidence in your own belief goes up with more data. But there are still plenty of cases where you can be sure even on the first shot.)