Sure he had views on the subject. But when he created his world, he set rules. So whatever were his beleifs, his world has to stay coherent. I repeat what i have said sometimes ago. One of the most important sins of Morgoth and Sauron was their desire to control the actions other beings, surely the curtailing of orcs' free will was merely another manifestation of the sin of domination.
Sin has a very specific meaning.
Orcs have free will when not under the control of Melkor or Sauron. Otherwise you could say the same for humans for Melkor also corrupted Men. You were already there when a few month ago we talked a bit about that. Yet ORcs have feelings, emotions. It is not impossible for them to have pity.