For most people, yes, the knowledge of some past lives could be very detrimental to their progression and health, but the sort of person to pursue this venture would probably be the sort who collects information objectively, and only learns from the past, whether good or bad, knowing it cannot be changed(easily). Of course, being Hitler, you would have a tremendous amount of karma to make up for...... but wouldn't the knowledge of that make you have something to strive for? A reason to do more good, to counter things in the past? Well, you should do as much good as you can really, but not everyone believes this, or follows this.