All in all, it all depends on viewpoints. The writer's, the translator's, and the reader's. There are different and conflicting versions of the story, but Valmiki's Ramayana and the Ramacharitmanas are taken to be the main ones.
There are three main types of versions of Ravan's Death.
- In which he wanted to be killed by Ram
- In which the Gods made a plan for him to be killed by Ram
- In which Rama wanted to rescue Sita, and Ravan was a... Uhh... Casualty.
All stories have these, or a combination thereof.
In version 1, where Ravan wanted to be killed by Ram, there are very few possible reasons I can think of. One would be, that Ram being an avatar of Vishnu, anyone killed by him would go to Vaikunth, the realm of Vishnu, which is basically heaven to the hundredth power. And talking realistically, I believe this rumor is because all the preachers and so wanted to save Ram from the crime of Brahma-Hatya-Paap, the crime of killing a Brahmin, which Ravan, being a son of a son of Brahma, technically is (Even though he performed a special puja to get rid of all that paap).
It is the second version which I think most likely, as there would be several reasons the Devas would want Ravan to be killed.
Ravan's father had two sons. Himself, a rakshas, and Kuber, a yaksha. Kuber was the treasurer of the Devas, and had a mongoose which would spit out jewels every time he opened his mouth. Due to this, Kuber was able to build himself an island city, entirely out of gold. It was named Lanka. Ravan came and conquered it, because he was jealous with Kuber for having it. So Kuber fled North, where he established Alanka, whose name gradually transformed into Alakapuri ( Puri means city ). So, Kuber had reason for wanting Ravan dead.
Ravana had obtained a boon from Brahma, which stated that no deva or asura or [insert name of all creatures and species] could kill or defeat him. It just happened that he did not say anything about Humans or Monkeys, whether out of arrogance, or forgetfulness (try reading out a hundred or two pages of a novel continuously, by memory) I don't remember. Now, characteristically, he started wreaking havoc on all three worlds. So, the average person or deva hated him.
Not to mention how many kingdoms he destroyed, and how long he had been destroying them. ( One hears stories of him being cursed in a fight with Ram's great-great-so-on-grandfather.)
And I can go on.
The third version, I believe, is pretty self explanatory.
As an additional tidbit pertaining to the third version, I would like to show how Ram respected Ravan for his knowledge.
LEARNING FROM RAVANA
Ravana lay on the ground, breathing heavily, waiting for death to come. ‘Quickly,’ said Ram to Lakshman, ‘go to him and seek out his knowledge. He knew a lot.’
So Lakshman went to Ravana and towering over him said, ‘I am Lakshman, brother of Ram, who has punished you for the crime of abducting his wife. As victor, he has a right to all that you possess, your knowledge included. If you have any honour, pass it on to him before you die.’
Ravana simply turned his face away, angering Lakshman, who reported the scene to Ram.
Ram said, ‘Here is a man who grabbed his brother’s house and another man’s wife and you expected him to just give you what you so rudely and authoritatively demanded as your right. You clearly never saw Ravana.’
Ram then discarded his weapons, walked up to Ravana, sat at his feet, joined his palms and spoke to Ravana in a gentle voice. ‘Noble one, son of Vishrava and Kaikesi, devotee of Shiva, brother of Surpanakha, Vibhishana and Kumbhakarna, father of Indrajit, uncle of Taranisen, friend of Mahiravana, husband of Mandodari, I salute you. I am Ram, who was responsible for mutilating your sister’s body, for which I have been duly punished. I am Ram, whose wife you abducted, for which you have been duly punished. We owe each other no debts. But I seek from you knowledge that you wish to leave behind as your legacy.’
Like a dying lamp restored to life with a fresh offering of oil, Ravana’s eyes lit up. ‘I realize I never saw you, Ram. I just saw the man who my sister hated, my brothers respected, my queens admired and Sita loved. In seeking knowledge from me, you are hoping that I will finally expand my mind and discover the essence of the Vedas, which has eluded me, even though I know all the hymns and all the rituals. You are the ideal student whose curiosity makes the teacher wiser. I bow to you. Brahma tells us that to receive we have to give but most of us, like Indra, seek to receive without giving. Shiva seeks nothing, so he does not bother with the accounts of giving or receiving, but only Ram, who is Vishnu, receives by simply giving. That is why Sita follows him, not me.’
Ravana then breathed his last.
Taken from Sita: An Illustrated Version of the Ramayana by Devdutt Pattnaik.
Because having to kill is different from wanting to kill.