I'm going to freak out at my aunt at my dads funeral
Why are you going to freak out at your aunt, especially at a time when everyone is mourning your father?
Because she just seems like some kind of bitch and it's hard not to get angry
Your aunt may not be a good person but that's no reason for you to lose control of your emotions. If you are that averse to her then you should ignore her.
It's fine, I'm angry and I just want to let all my emotions out.
Yeah, don't be reckless. There are times to do certain things, and a funeral event is not one of them. If you are angry, maybe find some kind of activity to do that can help you alleviate this anger, like something physical like boxing.