I saw the bridge was going to collapse, so I yelled at my stepmom to cross it
That seems like a bad, and dangerous idea.
I then watched from a safe distance as the bridge collapsed
It is wrong to have someone cross a dangerous bridge. They could be seriously hurt or injured if it collapses with them on it.
It's my family so it doesn't matter to you how i treat them
It is tough to get along with family and you do not have to be close with everyone in the family. That does not give you the right to put them in dangerous situation. I hope that your step mom will be okay and that you can make up with her.