When parents find out their children have head lice, they'll do almost anything to get rid of it. There are many myths out there as to how to kill lice, and one of the popular ones is mayonnaise. The truth is mayonnaise is not deadly, and it will not kill lice. The best way to treat lice is to use non-toxic, pesticide free natural enzymes that lice are not immunity to, and a really fine tooth comb.