Liars often deceive themselves so as to be more convincing to the people they lie to.
The victims of liars are driven by certain unstated premises, for example, the idea that nobody will want to harm themselves.
So a liar who has deceived himself will seem truthful, because his victims cannot find reasons for why the liar would attempt to hurt his own self by denying reality.
The phrase so as to I used above has to be interpreted loosely. Often conspiracies and deviousness can be pursued without conscious intentions. There is something within us which can drive us to act in ways to help us get what we truly want without knowing.