I share with my clients the idea that honesty is the sine qua non of a good relationship. A relationship based on dishonesty is not worth having because the feedback one gets from the partner is distorted, it is based on illusions and misunderstandings. The feedback has very little value. As they say in the computer field, "garbage in, garbage out."
Further, lying takes unnecessary emotional energy. The liars' anxiety goes up a bit because now they must expend the energy to keep the lie straight with other communications and/or to keep the truth in the dark. That lying is stressful for most people, and has physiological symptoms of stress, is the underlying assumption of how lie detectors work. These machines, supposedly, are able to detect these more subtle symptoms of stress which the human eye cannot detect.
Can people who lie be kind? At a superficial level, I suppose they can, but at a spiritual level it is very difficult because there is a pretense, a charade, and illusion created which is not authentic, not genuine, and comes between the people in the relationship.
In counseling I often hear the complaint, "He/she is not the man/woman I thought he/she was!" Or, "if I knew then what I know now, I would never have done it."
Learning the "truth" can be very disillusioning and often comes with a sense of betrayal. The dishonest presentation of self was not kind, but in the long wrong cruel, even if the cruelty was unintended.
Honesty is the best policy and in the long run is the kindest thing. Remember, being nice and being kind are not the same thing. You can be "nice" and lie your ass off - salespeople and politicians do it all the time, but being kind always requires honesty.
Honesty is also directly related to one's mental health. People who lie and equivocate loose track of who they really are, what they really think, how they really feel. They not only mystify others, they even mystify themselves and they loose their authentic voice. As Polonius says in Hamlet, "This above all: to thine own self be true, and it must follow, as the night the day, thou canst not be false to any man."