Moral evil is when someone does something that they know is wrong, or when they don't do something that they know is right, on purpose. It's when people make choices that hurt other people, animals or the environment, or when they don't help people who need it.
For example, if someone steals from their friend, they know it's wrong and they do it anyway, that's moral evil. If someone sees someone else getting hurt and they don't try to help, they're also doing something wrong. This is because they're choosing to be mean or not help, even though they know they should.
Now, sometimes people make mistakes and they don't know what they're doing is wrong, that's not moral evil. But when someone does something they know is wrong, it's important that we teach them why it's wrong and how to make better choices in the future.
It's important to be good and kind to others, and not do things that we know are wrong. That's called being morally responsible, and it helps make the world a better place for everyone.