I'm mad at my grandparents
Is there a reason you would be mad at them?
They have been always nasty to my mum
I'm sorry you are feeling that way. Family members are not expected to get angry with each other. They may need to sit down and talk things out with each other, but as long as they aren't treating you that way, you shouldn't be mean towards them.
It doesn't matter if they weren't mean to me. I am going to show up at their house and be mean to them as much as they did to my mum.
That won't solve anything and you would make the situation worse. You don't have to talk to them. In fact, it would be better if you avoid them all together until the situation settles.