> Evil can act Good, but Good cannot act Evil. Evil has that advantage over Good that cannot be matched.

The assumption here is that being Evil is advantageous. It's obviously advantageous to personal gain, but the relevant thing is whether it's advantageous to staying in power, and then the system you want to design is the one where it isn't.

Notice also that it isn't binary. You can make it harder to be Evil and then have less of it even if you never have none.