We're all taught telling the truth is the best path. Truth is good, etc. The reality of day to day life is different. We all lie. We just justify it in different ways. "If I told her the truth, I would hurt her feelings," or "It's not a big lie," or any number of other ways. It's a long accepted fact that very few people want an honest answer to the question "Does this outfit make me look fat?" if that answer would be a resounding "yes".
My characters lie all the time. Some lie because of their own agendas. Some lie because of societal pressures. Some just like to lie for fun. Often, they lie because they're afraid.
There's also the issue of self-honesty. How often do we lie to ourselves? Some people are able to take a step back and view themselves honestly, even when they know the answers might not be the most flattering. They accept their motives weren't the purest. Others can't do this. They have to see themselves as good, regardless of what their actions may say.
My characters suffer from the same dilemma. Some see a true reflection in the mirror and others' view is distorted. I think this makes them all a bit more human and, hopefully, a bit more relatable to readers.
No one is pure evil (well, some are). No one is pure good. No one tells the truth all the time. Anyone in a relationship knows a few little white lies makes certain days go smoother. Even people with the best possible intentions can roll in the mud once in a while.
It's been my experience that when someone says "I did it for the best" usually means they've done something questionable. Maybe it was for the best. Maybe it was for their best.
Are your characters truth-tellers all the time? Maybe they should lie. Just a little.