How to improve and get better by using taboos and your dark side seems like a contradiction. But sometimes using your dark side can be just the thing you need to improve.
Now this can be harder to do than you might suppose. Our culture (from Faust to Star Wars) says appealing to the dark side leads to your ruin. This can cause you to not want to do this. But anger (against injustice), pride (in becoming the best), gluttony (for hard work) and lust/vanity (to self improve) can sometimes motive you and drive you much better than love and happiness can. Sure, excessive vices can destroy you, I agree. But vices well harnessed and in moderation might get you through the difficult days of changing and help get you to the other side.
To see a good example of what I mean, read this: A Spiteful Guide to Self-Improvement.
You may not have an enemies or rivals or arch enemies. Fine. Invent some. Then go out and take them on and in the process become a better person. Then be magnanimous and graceful in your victory. There: now you are a better person and a good one too.
(Image by Gustave Doré, Jacob Wrestling with the Angel (1855) from wikipedia)