Can I slap my grandpa?
I hope you are joking. What makes you want to slap your grandpa?
I hate him, I will do that at upcoming family meeting
Hitting family members is rude. If you hate him, you can try and talk to him about it, or you can just ignore him. Violence is not the answer here.
I did. I tried talking to him and he never listens to me. He thinks he can do anything and it's okay.
Well if you already tried talking to him and things haven't changed then you can cut him out of your life. That would be better than hitting an old man.