Nice trick, thank you for sharing ! One little reserve: at least in C, this fails if the code to toggle contains comments itself:
slay_the_unicorns(); /* FIXME: This function kills unicorns ! */
In this code, you won't kill unicorns, but you will always launch a missile, which is not what was intented.
It only works in C/C++, but I tend to prefer:
slay_the_unicorns(); /* FIXME: please don't to that. */
You just have to change "1" to "0" to toggle between nice and evil code, and it has no restriction regarding embedded comments.