The universe seemed to always be made up of two sides. If there was light, there would be darkness. If there was Yin, there would be yang. If there was good, there would be evil! A cultivator's cultivation had to be true to their heart, which was to do as they pleased. However, no cultivator could really do as they pleased. That was why in the process of cultivation, the heavenly Tribulation would appear, and the inner demons would appear.
Cultivators had many ways to deal with inner demons, but all of them were to suppress them. Suppressing the inner demons wouldn't make them disappear. Moreover, the longer you cultivate, the more inner demons you'll accumulate. At that time, the inner demons you'll have to face will become stronger and stronger.
Many cultivators thought that their inner demons would become stronger because their cultivation had increased. In fact, that wasn't the case. It was because you suppressed your inner demons too much that they became stronger.