Just because the chirps SOUND aggressive/angry DOES NOT mean that they are agressive or angry at all.
Please remember, ALL birds are trainable and can be tamed!
Different chirps mean different things...
There is warbling, which is a soft consistent chirp.
There's a loud chirp, which sounds aggressive, but is really not. It sounds like, "Ack! Ack! Ack!" (Is this the sound you're referring to?) Budgies are not generally mean. Most birds are aggressive out of fear. But just because she/he makes these sounds doesn't mean she/he is mean! It is just being a budgie!
One of my budgies makes this sort of sound, but he is definately not aggressive. He's either scared or having a little tiff with one of the other budgies, but it doesn't last long.
Please remember, birds don't want to be mean. I've never met a bird with an unfriendly personality. I have, however, been involved with birds who have been very fearful and have made that "Ack" screech.
I would not return him/her. Give the budgie a chance! : )
I hope this has helped.