I did something very bad once, and I was very mean. After that, I went through periods of sadness, and I went through some very hard things. I was only about 11 years old then, didn't know any better, and I feel very bad about it even today. I have said sorry to them though.
I think what people need to realize is to be kind to everyone. You don't know what they've been through, or what they're going through, and the biggest thing you can do is to be there and to be kind to them. Imagine what a difference you could make; no matter how small.