I took a dump in my brothers guitar case
Did he do something that made you want to take a dump in his guitar suit case?
Yeah he is awful at playing music.
It's disgusting that you did that to your sibling. It's wrong and you shouldn't crap in other people's stuff as it was a very immature and childish move on your part. Now you have to consider what he will do to get back at you, hence the cycle begins.
What? He has to clean it up. It's his fault.
No, it is solely your fault. You could have tried to help him find lessons or youtube videos, they also make headphones where he can practice in silence. Pooping as revenge is animalistic and weird, go clean it up.