People's attitudes can be changed by changing their behavior. Get someone to do something, and they'll rationalize why they did so if they can't think of a good reason. Get someone to do something that distresses them, and they'll rationalize very strongly, especially if their self-image isn't compatible with a negative assessment of the action.

Think of really harmful hazing. If no such tradition existed, people wouldn't react well to someone trying to start it. Once people go along with minor hazing, there's less of a psychological barrier against it and more of a barrier against viewing it as bad. It then becomes easier to progress to more serious hazing. Finally people try to force others to do really stupid, risky, or even certainly-harmful things, while never really considering the costs or consequences.

People are consequentialists. If a consequence of believing X "that an action is harmful" is to conclude that they've done harm, people will tend to deny the possibility of X.