[¯|_(ツ)_|¯]

Kids get really curious sometimes, and sometimes they stumble upon things that they shouldn't. No matter how much a parent or sibling tries, sometimes it's hard to shelter them from these things forever. It's not like you can watch them 100% of the time, and sometimes you do have to leave the room. You did the best you could by shutting off the movie when you realized what he was watching.

Maybe give your sibling some time to cool off. It's okay and valid that he is upset, since the scenes in some movies can definitely be uncomfortable, but eventually he should be able to move past it. When you've given it some time, you can sit him down and let him know that you're sorry that you weren't there to tell him not to watch the movie, and remind him that he is not a bad person for knowing these words now. He definitely doesn't have to use the words that he learned. Remind him that it was a mistake on both of your parts but it's something that's a learning experience for both of you!

Accidents happen so I wouldn't beat yourself up too much. You were busy with other things, and that's okay. Sometimes people have a lot on their minds.

Maybe you can also talk to your mom about what happened. You said your mom told you that your brother is mad, so maybe she can give you some advice on how to deal with him?