I watched as someone dropped their wallet on the ground, then I went behind them and kept everything
That's theft! I hope you're not going to keep it.
My friend and I were in a strange mood and we decided it was funny.
It is wrong to steal from someone if they accidentally dropped something. It is a crime to spend their money or use their id's.
It was just a joke.
If it was, then did you bring it back to them with everything intact? Theft is a very serious thing, wouldn't you feel bad for taking away their livelihood? It would be best to try to contact the owner and give it back right away.