Silmarillion - Chapter 24 - Of the Voyage of Eärendil and the War of Wrath

[Inziladun]

Originally Posted by Mithadan

Absent from the published Silmarillion are a few lines found in Morgoth's Ring (don't recall which section, either the Annals of Aman or the later Silmarillion) regarding Morgoth's release from Mandos. Manwe is described as being without evil and being unable to understand it. As a result, he believed that Morgoth could be redeemed or rehabilitated.

The same error seems to have been made regarding Sauron after Morgoth's defeat.

Indeed. Giving second chances was apparently hardwired into the Valar. Remember too that Gandalf was ready to pardon and allow Saruman to repent, though in that case an imperfect and merely temporary atonement would seem to have carried much more risk than a backsliding Sauron.