@ Ramya, by and large all the asuras were the great devotees of Shiva. Just imagine what would have been the level of Bhakthi of Ravan's mother who could prepare a shivling by using just the sand and do Abhishek and all, and still it never gave away (collapsed due to water & milk poured during Abhishek).
As we have heard that death doesn't happen without reason means yama being the custodian of Dharma will not take away anyone's life without a reason. And he never tried to take away the lives of Shiv bhakta's unless, Shiv give way for him to do his duty otherwise he had to go through Shiv ji , we have known the of Markandeya where Lord Shiv has to make amends with Yamaraj. hence it is better to give away some boon and boost the ego of his bhakth's so as to make them falter and then give reason for their downfall and end. So if not for his boosted ego, Ravan would not have been defeated by any.
Had it not been fro his ego that he is the great Shiv Bhakth and it is better to have the Shiv Dhanish with him (the great ruler) & in his country than With king Janak who rules a comparatively smaller province. So at that moment his main interest was to gain the Bow and not Sita as he was not a spinster to dream about a girl. Even later, if he was ill at nature and wanted to avenge Sita for his failure in Swayamvar, he could have donated her to any of his brothers or children (the way Bhishma did with Amba, Ambhike & Ambhalike)and there wa sno need fro him to wait for her consent to have her (about the bane given by Nalkubher).
I don't think his intention was to have Sita for him but to show to Ram that he could take away his wife under his nose and Ram could not take care of his own wife of whom he won through the Swayamvar by breaking the Shiv Dhanush.
