Is it bad that I just wrote all over my brother's car windows in soap?
What led you to write all over your brother's car windows with soap?
I was fucked up on drugs, I must have been crazy
Even though you were on drugs at the time, you should not mess with someone's property by writing on their car. Especially your brother's car because siblings need to respect each other and their property.
But it was only soap. It will probably come off with water.
It's true that it will probably wash off, but you still shouldn't have done it because vandalizing your brother's car isn't a good thing to do. It's never right to mess with someone's personal property. I think you should volunteer to clean up the mess you made, so your brother doesn't have to.