Why Ravana of Ramayana is worshipped like god and also considered as a demon? I know he was a great king who kept his kingdom and administration in an organized manner. He was also a Shiv bhakt. But still he is considered as a demon in Ramayana because he kidnapped Sita.
If we go by Uttar Kanda of Valmiki Ramayana then his act of Kidnapping Sita is also considered not bad. It is told he did it to get liberation from hand of Rama. The Story is in Section XLIII of Uttar Kanda of Valmiki Ramayana:
Agastya Rishi is describing stories to Rama. Now he says:
O king. I shall now again relate to thee another divine theme. O RAma I shall now describe to thee why did Ravana steal Sita ? Do hear it attentively.
Then Agastya describes story of Ravana meeting with the great Sage Sanatkumara. He questions about liberation to the sage.
“Being slain by Hari to what position do the Daityas, Danavas and Rakshasas attain? And why do Hari destroy them ?” Then there Sanatkumar replied :—“ Whoever are slain by the celestials do reside in their region. And being banished there from. they again take their birth on earth. Thus by the misery and apiness acquired in their pristine existence, they go ough births and deaths. O king, all the monarchs, slained by Janarddana,holding discus and the lord of the triple world, ained to his own region; even his anger becometh like a boon."
Then after hearing this Ravana began to think how should he plan to be killed by Hari.
Hearing those words given vent to bythe great ascetic Sanatkumar the night-ranger Ravana, being worked up with delight and surprise, began to meditate how he should enter into conflict with Hari.
Then from Sanatkumara he gains information that Hari will be born in Treta in Ikshvaku dynasty as Rama. Sanatkumar also tells about Rama will be abandoned from Kingdom and come to Dandaka forest. Then he plans to Kidnap Sita so that Rama may kill and liberate him, so that not only he, all his clans will be liberated.
the vicious souled Ravana stole away Sita with a view to meet death at thy hands. Thou art NarAyana, holding discus, conch and club ; thou hast in thy hands....
Thus, from above we can know Ravana was a great devotee of Hari, Hara (Shiva) and Brahma too.. But this story remains as hidden secret in other Kandas. In other Kandas act of Kidnapping Sita is described as Ravan being overcomed by Tamastatwa.
So, it depends on you how you consider Ravana. As a devotee or as an evil.
In Sundara Kanda of Valmiki Ramayana, Hanuman addressing Ravana says the following verses which I think sum up Ravana's life:
न तु धर्म उपसम्हारम् अधर्म फल सम्हितम् || ५-५१-२८
तत् एव फलम् अन्वेति धर्मः च अधर्म नाशनः |
The fruit of righteousness does not exist together with the result of unrighteousness (even though he practices virtue). That consequence of unrighteousness alone follows him. Righteousness in abundance destroys unrighteousness.
प्राप्तम् धर्म फलम् तावत् भवता न अत्र संशयः || ५-५१-२९
फलम् अस्य अपि अधर्मस्य क्षिप्रम् एव प्रपत्स्यसे |
You already obtained the fruit of virtue till now. There is no doubt about it. In no time, you will obtain the fruit of unrighteousness also.
Now those who consider Ravana a hero might be reveling in some of his so-called virtues, e.g.:
His penance to get boons from Lord Brahma.
But this was motivated by jealousy towards his stepbrother Kubera as explained here.
He singing praises for Lord Shiva.
However, he did this only on the advice of his ministers when his arms were trapped under Mt. Kailash and there was nothing else he could do to free himself.
I know he was a great king who kept his kingdom and administration in an organized manner.
I suppose that's because he consulted and listened to his ministers early on and was riding on good fortune and the boons he received from lords Brahma and Shiva.
धर्ममर्थं च कामं च पृथिवीं चापि लक्ष्मण |
इच्छामि भवतामर्थे एतत् प्रतिशृणोमि ते || २-९७-५
O, Lakshmana! It is for your sake I desire virtue, legitimately acquired wealth and pleasure or even the earth itself. I promise this to you.
भ्रातृइणां संग्रहार्थं च सुखार्थं चापि लक्ष्मण |
राज्यमप्यहमिच्छामि सत्येनायुधमालभे || २-९७-६
O, Lakshmana! I desire the throne only for the protection and happiness of my brothers. I touch my weapon and swear this fact.
Long before Ravana kidnaps Sita, he rapes Rambha and also molests Vedavati. Both his brothers, Kumbakarna and Vibhishana question his actions after Sita's kidnapping and also advice him to release her and save both Lanka and its people.
Hearing the lamentation of Ravana, Kumbhakarna laughed at a full volume and spoke as follows: "The same bad consequence, which was imagined by us earlier while taking the final decision after our consultation, is faced by you now, due to your utter disregard of the words spoken by your well-wishers."
Of course Ravana doesn't listen to them. Poor Kumbakarna decides to side with Ravana out of loyalty. Vibhishana however chooses dharma over his brother and seeks Rama's help and becomes instrumental in Ravana's downfall.
All the above just goes on to show what a bad person and a bad king Ravana is.
That's why most Hindus and adherents of Valmiki Ramayana don't consider Ravana a god nor do they worship him. In fact, another name given to Ramayana by Valmiki is poulastya vadha (Ravana's elimination).
However, there are some people in India like the Gond tribals of Maharashtra as mentioned in The Hindu article, who don't consider themselves Hindus nor do they believe in Valmiki Ramayana, so it's understandable they consider Ravana their god.
“Our understanding of Dussehra is different. It doesn’t signify the slaying of Ravan but implies ‘Dassar Puja’, or the worship of weapons,” says Vasudevrao Tekam, a descendant of the Gond king of Lanjhigarh in Odisha. Tekam was among the many Gond guests present at the function. “We have nothing to do with Hinduism. Gonds believe in totems. We are the warisdars of Ravan: Ravanvanshis. We are not Hindus. We are being forced to be Hindus...” Tekam’s disowning of Aryan terminologies is indicative of the Gonds’ counter-narrative.
According to their version, Ravan was a Gond king who was slain by Aryan invaders. He was the tenth dharmaguru of the tribe, carrying forward the legacy of Kupar Lingo, the supreme deity and heroic ancestor of Gonds, who gave them their phratry structures and lifestyle values.
Also, A. K. Ramanujan says the following in his essay Three Hundred Ramayanas:
When we enter the world of Jains' tellings, the Rama story no longer carries Hindu values. Indeed the Jaina texts express the feeling that the Hindus, especially the Brahmins, have maligned Ravana, made him into a villain.
Vimalasuri the Jains opens the story not with Rama's genealogy and greatness, but with Ravana's. Ravana is one of the sixty-three leaders or salakapurusas of the Jaina tradition. He is noble, learned, earns all his magical powers and weapons through austerities (tapas), and is a devotee of Jaina masters. To please one of them, he even takes a vow that he will not touch any unwilling woman.
So depending on which Ramayana one follows one could end up viewing Ravana as their hero or villain.
Valmiki Ramayana - Answering Pertinent Questions with Shatavadhani Dr. R Ganesh (YouTube playlist)