Was Ravana actually a villain?
Yes, he was. If by villain we mean someone who trades the path of unrighteousness then Ravana was nothing less than a villain:
जप जोग बिरागा तप मख भागा श्रवन सुनइ दससीसा।
आपुनु उठि धावइ रहै न पावइ धरि सब घालइ खीसा।।
अस भ्रष्ट अचारा भा संसारा धर्म सुनिअ नहि काना।
तेहि बहुबिधि त्रासइ देस निकासइ जो कह बेद पुराना।। [RCM - 1.182]
Ravana would get up and run at once to stop austerity, penance, dispassion, etc. as soon as any talk of it entered in his ears. Such prevalent was corruption and sinful wrong actions that dharma was no where to be heard of. He would torture in many ways and sent to exile whoever read the Vedas and puranas.
Was Ravana a devotee of Lord Vishnu?
No, he was not. He was a devotee, or better a worshiper, of Lord Shiva and Brahma like you have stated. He did this to acquire power and boons which he used to spread terror and havoc on people. No devotee can ever imagine hurting his deity even in his dream, let alone stealing the wife of his deity and fighting with him. The definition of a devotee is someone who thinks in favour of his deity:
anukulyena krsnanu-silanam bhaktir uttama [BRS - 1.1.11]
Devoid of any personal desires, not tainted with impersonal knowledge and fruitive actions, following of Krishna in favour of Him is known as the best of all devotions.
But Ravana's actions caused difficulties and hardship for Shir Ram. No devotee can do that. So he was not a devotee of Vishnu, otherwise he wouldn't have developed such an enmity towards His human incarnation.
It is a rare privilege to be killed by the hands of God, Isn't it?
Yes, it is. Ravana got the privilege which is even rare to get for the saints and sages:
खल मल धाम काम रत रावन। गति पाई जो मुनिबर पाव न।। [RCM - 6.113.5]
- The wicked, sinful, lustful Ravana attained an exalted state even rare and scarce to the sages.
So now we need to do the thinking. Can any ordinary soul compel and make the the supreme Lord to descend on earth for him? Can any ordinary soul dare to take away Maa Sita, the wife of Shri Rama, from the Lord Himself? Can any ordinary soul get the privilage of getting killed in the hands of the Lord Himself? The plain answer is, no.
Story behind the scene
So Ravana was no ordinary soul. He was previously one of the two gatekeepers at Vishnu's abode Vaikuntha. Due to stopping the four Sanat kumaras they got cursed and had to take birth in demonic families as the Lord had desired:
The Lord replied: O brāhmaṇas, know that the punishment you inflicted on them was originally ordained by Me, and therefore they will fall to a birth in a demoniac family. But they will be firmly united with Me in thought through mental concentration intensified by anger, and they will return to My presence shortly. [SB - 3.6.26]
So Ravana's activities were of a villain, but it was only to render a pastime for the Lord. So the moral of the story is that, even the greatest enemies of the Lord are nothing but His true devotees. And Ravana in his life did nothing but devotion to Vishnu in the mode of enmity. It is because, due to enmity and anger he always remembered and thought of Shri Rama. And we know thinking of the Lord in any way is nothing but an act of devotion and it causes oneness with Him:
kāmaṁ krodhaṁ bhayaṁ sneham aikyaṁ sauhṛdam eva ca
nityaṁ harau vidadhato yānti tan-mayatāṁ hi te [SB - 10.29.15]
Persons who constantly direct their lust, anger, fear, protective affection, feeling of oneness and friendship toward Lord Hari are sure to become absorbed in Him.
This type of devotion out of anger and fear is known as pratīpa bhakti (BRS- 2.3.92). But it only suits such powerful personalities like Kamsa, Ravana, Sisupala, etc. who behind the scene are nothing but Vishnu's personal servants and devotees.