For all his alleged wrong doings Ravana was regarded as the greatest devotee of Lord Shiva and the most knowledgeable in all the 64 forms of art/science and a hugely respected and just ruler among his people.
I can only think of one line I read in the superb book "Immortals of Meluha" to describe Ravana - Just because they are different does not mean they are evil.. Ravana was different, not necessarily evil.
Ravana, was indeed, a scholar of sciences, well-versed in Vedas, highly intelligent, mighty and formidable. He was feared by Gods, sages, humans and many other races. He followed, rigorously, rituals pertaining to deities. He had performed a tremendous tapasya for a 10,000 years sacrificing his 10 heads every 1000 years. He obtained a boon of invincibility from all races, excluding Humans and half-humans(Vanaras), granting him a virtual immortality.
However, despite his majestic qualities, it is also true that he had several despicable traits. The chief flaw in his character was his fatal attraction to other women. Even though he was married more than once, he never failed in pursuing other damsels. He waged many wars and forcibly abducted innocent women and enjoyed them. He even raped the wife of his step-brother's son.
Ravana, was already cursed by Nandi(the vehicle of Lord Shiva) and Goddess Parvati(consort of Lord Shiva).
Ravana was, infact, punished by Lord Shiva himself for the impropriety. Ravana was so full of himself that he disregarded Lord Shiva and tried to lift Mount Kailasa on his 10 heads. Lord Shiva punished the insolence by crushing the 10 heads of Ravana under the Kailasa. He remained in this pathetic position for several years and ultimately on the advice of his ministers he composed a poem in extolling Lord Shiva. It was, then, that he was freed by the Lord. Lord Shiva also gave a title 'Ravana' to him on that occassion. Ravana means one who makes terrible laments. Ravana made terrible lamentations when he was crushed under the Mount and hence he was bestowed with this title. His original name, given by his father, was Dashkantha(ten-headed one).
Ravana sorrounded himself by sycophants and never listened to wise counsel. His nature was cruel and he was extremely selfish. He did not mind trampling on other's wishes to fulfill his own narrow ambitions.
Boons blinded him and he became arrogant. He suffered several setbacks also due to his smugness. He was famously defeated by Vali(brother of Sugreeva and King of Vanaras). He was losing the battle with Gods, when his son Meghanath saved the day by capturing Indra(King of Gods). As a consequence, Meghanath earned the title of 'Indrajith' and a boon that granted him great invincibility.
Ravana desired others' properties and did not hesitate from snatching them on the strength of his might. He looted the world and enriched himself. Pushpaka Vimana was looted from his step-brother. Infact, Lanka itself was earlier ruled by Kubera(Ravana's step-brother) but he was driven out by Ravana and Lanka came under the sway of demons.
Ravana ruled for long. He was beneficial to his subjects, though he did not tolerate dissent or even diverse views. Lanka flourished under his regime and became the hub of all riches. Various kinds of wicked and perverted demons gained great power under his rein. They extracted sadistic pleasure from injuring and harming the innocent, weak and virtous.
Ultimately, he abducted the virtuous Sita Devi and harassed her. That was the proverbial straw that broke the camel's back.