It was her birthday, so I set the colorful birthday decorations on fire.
On her birthday? Why would you set her decorations on fire?
I was mad at her and wanted to get back at her.
It's wrong to set fire to colorful birthday decorations that someone has put up. Destroying something to get back at someone is unethical and mean. You need to control your temper.
But she's such a child! She can be so annoying sometimes.
Just because someone annoys you doesn't make it okay to destroy their things, especially ones that someone put effort into for them. It's not her fault you find her annoying. How would you feel if someone found you annoying and destroyed something important to you?