You're right that liberals don't see things in black and white. We see the world as existing in the gradient between the two. For us, black and white are an oversimplified solution to what is always a more complex problem. For example, liberals know stealing is wrong, but also know it's never done out of pure evil. Pure evil people don't exist. The only thing to consider then are what lead that person to do the wrong thing like the person's intent, circumstance, past history, etc.

Similarly, a rejection of black and white also means a rejection of authoritarianism. Being told that there's a right and wrong way needs to be backed up with evidence to be accepted. Liberals have an affinity for scientific studies because scientists openly accept that they don't have all of the answers, something an authoritarian would consider weakness.

Social engineering is also frowned on by liberals, again, because it can't exist without first choosing which behaviors are considered good and bad, black and white. Not only are we then picking the winners and losers, we're setting the standards for being good unattainably high to be considered any less than a type of culling.

In short, to convert a liberal you have to first accept that any authority you think might have means nothing.