You know the answer.
It’s because, in their minds, the ends justify the means! Doing unethical things in the name of what they consider to be the “greater good” is not just okay, it’s admirable.
And, having known pathological liars, they all believe that what isn’t known, hurts no one. This sort of person will actually justify a lie as a beneficent act that saves others from knowing something hurtful.
They lie out of kindness.