I've posted much of what follows in the
Music in middle-Earth thread, but feel that it's also relevant here.
Sauron is an enemy; but he is an enemy both immortal and of divine origin. Not only can he live forever; it appears that he cannot be 'killed' in any conventional sense. This makes him thousands of times more formidable than any human tyrant. This did make a difference, such as when the Mongols looked as if they would overrun Western Europe, they were stopped by the death of the Great Khan Ögedei in 1241. In Sauron's case, not only do you have the same person around; that person can afford to wait a long time for things to turn in his favour. He waited for centuries in the Third Age until Gondor began to decline and the watch it kept on Mordor ended. The Gondorians, Rohirrim and other enemies can't wait, as they would for a mortal foe, for him to either die, or to grow old and want to spend time with the grandchildren.
There's also the problem of what to do with Sauron if, by a remote chance, he was defeated. Keeping him as a POW would be too risky, considering what he did in Númenor in the Second Age. All he needs to do is to wait a couple of generations for those who knew him as an enemy to die off, and let people grow up who might feel sorry for him. We can also look back and see the havoc his former master, Morgoth, wreaked in Valinor after he was released. The only way to keep him harmless when he was overcome again at the end of the First Age was to imprison him outside of Arda, casting him out beyond the Walls of Night.
Even if Sauron is not imprisoned, but a decision made to execute him instead, is such a thing possible? Can a Maia (or former Maia) be killed?
There are two disadvantages to being an immortal tyrant. First, you have accumulated a huge number of atrocities to your name over thousands of years.
(I'm sure that Sauron would have made Mao and Stalin look pathetic by comparison.) You're therefore easy to hate. For example, I'm sure that the Gondorians haven't forgotten the betrayal of their last king.
Second, because you've been around so long, your enemies will have amassed a large amount of information about you. Know your enemy is an basic maxim of warfare. I believe that Aragorn II learnt a lot about Sauron, from written and oral sources, as well as from his own travels, and was thus able to use that knowledge to confront him using the Stone of Orthanc, and persuade him that he might have the Ring, encouraging him to make a premature attack on Gondor.