What is kill fee?
An agreed payment if a client cancels a project part-way through.
A kill fee is what you are owed when work is cancelled through no fault of yours. Usually it is everything completed to date plus a percentage of the remaining fee, or a non-refundable deposit.
Without one, a cancellation halfway through is a negotiation held at the worst possible moment. With one, it is a calculation, and both sides already agreed to it when nobody was upset.