Here is a truly profound quote by Bertrand Russell:
"The conception of Sin which is bound up with Christian ethics is one that does an extraordinary amount of harm, since it affords people an outlet for their sadism which they believe to be legitimate, and even noble."
People have natural cruelty in them, and they also have a natural desire to view themselves as good. The concept of sin allows them to satisfy both; they get to indulge their cruelty by punishing the sinner or by cheering the punishment from the sidelines and at the same time they get to retain their belief in their own goodness because the concept of sin has built into it the idea that the sinner had it coming or even that the punishment was for the sinner’s own good. That doesn’t mean that there is no genuinely evil behavior deserving of condemnation and punishment, but the existence of this really nasty bias ought to make one set the bar for doing so pretty darn high. Here’s another by David Brin:
"While there are many drawbacks, self-righteousness can also be heady, seductive, and even… well… addictive. Any truly honest person will admit that the state feels good. The pleasure of knowing, with subjective certainty, that you are right and your opponents are deeply, despicably wrong.."
Allowing yourself to enjoy your own rightness and the other guy’s wrongness might have some merit if it is something that you give yourself as a reward for fighting genuine injustice. But the fact that it is so much fun (Brin believes that it is literally addictive in the brain chemistry sense) ought to make you very suspicious of it; if it’s that much fun you are going to want to adopt the beliefs that allow you to get it in its purest and tastiest form, and such beliefs are unlikely to correspond to truth.