I used to write inappropriate stuff and post it to random people's Facebook accounts who would forget to log off from the phones on display in the store.
Can you tell me about the inappropriate stuff you would write?
I'd make hacky dark jokes and I'd make fun of strangers' pictures and statuses.
Even though the people who used the phones on display should have been more careful, it was wrong to try to use their accounts and try to impersonate them for your own amusement. Trolling others under a false identity will just cause a lot of grief and trouble for the person being impersonated. You should be more respectful of other people's accounts as posting inappropriate things can cause a lot of social and reputational damage.
Man can't you take a joke.
It's not appropriate even as a joke. Maybe you don't do it in real your ideas are evil. You should focus on doing something constructive rather than having these kind of thoughts.