People might start out intending to be honest, but it is incredibly easy to deceive yourself (and often very easy to deceive others, too). People have an amazing ability to convince themselves that they are being honest, or that their lies are white or justifiable, no matter how objectively terrible their acts may be. There are a variety of reasons for this, starting with the fact that humans are social animals and social life requires lies ("Does this dress look good? Am I getting fat?"). If you're interested, there are a range of good psychological studies on cognitive dissonance, one of the effects causing people to be less than honest.