In a switch where the contacts remain in one state unless actuated, the contacts can either be normally open until closed by operation of the switch, or normally closed and opened by the switch action. Basically, depending on the system the ATV uses, the kill switch would need to either open the circuit to kill the engine(requires a normally closed switch) or would need to complete the circuit to kill the engine( requires a normally open switch). This all depends on what ATV you have to determine which style you would need.
A normally closed switch has a completed circuit under running conditions and opens the circuit to kill the engine. The normally open switch does not have a complete circuit under running conditions and will close the circuit to kill the engine. Which one you need will vary depending on what model it is for.