What is 'Kill'
To cancel a trade or order that has been placed, but not filled. A trader or investor may desire to kill an unfilled order when the market has moved against him or her (or merely because he or she has changed his or her mind), especially if the order is a market order rather than a limit order. Given the split seconds with which orders are executed today, a kill command needs to be sent almost instantly after the buy or sell order has been transmitted. Even then, there is no guarantee that the original order will be killed.
It may be especially difficult to kill unfilled orders in volatile markets with heavy volumes. This is because exchanges are sometimes unable to handle exceptionally heavy volumes; in such situations, the trader may be unable to receive confirmation of whether his or her original order was filled, or whether it has been killed. Note that the trader or investor is liable for the order once it has been filled.