👉 Due to bad karma, fallen ascetic became demons

Ravana was a wise man. Through his austerities he had acquired great capabilities. In spite of all this, due to his abominable behavior, God himself had to take birth on the planet to kill him. Example of Bhasmasur, illustrates that even after having reached a high spiritual state, without any compellations, due to ill state of mind one digs a hole for himself. Similarly, great powerful fighters like Hiranyaksh and Hiranyakashyapu had to die due to evil state of mind and behavior. 

Durga killed Shumbh-Nishumbh, Madhukaitabh and Mahishasur. Parshuramji killed Sahasrabahu and Shri Krishna killed Vanasur. Bhim killed Jarasandh. Shoorpnakha used to tease Rishis performing austerities in Khardooshan forest. Tadka used to disturb the yagyas. Bali had out thrown his younger brother and kept his wife with him. Ram and Lakshman killed all of them and ended their tyranny.

Pragya Puran Part 1