When his grandparents sleep, I'm going to tear their laundry.
Why would you try to sabotage their laundry?
The old man seriously pisses me off just by being there.
Ruining your friends grandparents clothes is extremely rude. If you have an issue of your friends grandparents you should talk to your friend about it. If you end up tearing up their possessions than most likely your friend will no longer talk to you any longer.
Nah, the old man is mad too since he always taunts me to do it.
It does not belong to you though, so you should not touch it. How would you feel if someone ruined your clothes while you were asleep?