If you kill a hawk when there's no good reason to kill a hawk then you could get a fine of $10,000.00 to $15,000.00 and possibly 6 months to a year in jail or prison.
But if you do need to kill the hawk because it's attacking your chickens or animals then you usually will not get into trouble for it.
If you're having trouble with hawks trying to attack your chickens or animals contact the Game Warden or wildlife people in your area and ask to get a permit for killing the hawk when needed and you certainly won't get into trouble for it.
Some places if it's your first offense at killing a hawk you might only get a $50.00 to $200.00 fine and no jail time or prison time.
It depends on the state.