My first piece of advice, Hallann, is to choose your fights wisely in the future. The Cenarion Circle is devoted to preserving harmony and balance throughout the world of Azeroth; they do not take kindly to those who slaughter public servants within their cities, as you have seen by how your standing with them fell when you killed that flightmaster.
That said, it may be possible to pursue restoration for the reputation lost because of this act; if you submit an in-game petition describing your plight, your situation shall be given further review. However, please bear in mind that we may offer no guarantees that your request shall be granted, and that reimbursement is never certain. I wish you the best. :)
Look at my works, ye mighty, and despair...