To clean the switch contacts – it might help to rock the switch back and forth few times. If that’s does not fix it, spray some contact cleaner in the gap between the switch button and the housing and work the switch on and off 10 or 20 times.
If that does not help, you could take the switch apart to clean it, or buy a new switch.
Some people solve the problem by not using kill switch.
This is uncommon, but if switching the kill switch off and on several times resolves the issue, it could be a bad CDI unit.
Rebel500 