The Meaning of Pity and Mercy...what is the RIGHT thing to do?

[Kuruharan]

He can somehow look deep into Gollum's small, shriveled heart and see there light, still hidden, almost extinguished, but still the possibility of goodness.

I realize that it is not particularly relevant to this thread, but Gollum did not do what he did to save the world out of goodness.

does the end justify the means? Say that you kill someone either to save yourself, or to prevent them from hurting others in the future. Does the fact that you killed for a good cause justify the fact that you have taken away a human life? Furthermore, if you kill someone to save yourself, are you not implying that your life is worth more than theirs is, because you should live and they should die?

Now you come to an interesting problem of the human condition. We don't know everything, we are not everywhere, we don't understand all that is going on, but we still have to live and act and make difficult choices and then live with the consequences of those actions.
I suppose that is really an unanswerable question.

It's worth considering that both pity and mercy are the prerogative of those with some kind of power over the potential recipient.

But what exactly do you think that Bilbo's and Frodo's position in relation to Gollum was? (Sticking strictly to the context of the story in this case.)
In one case, Bilbo was invisible with a sword and was in a position to kill Gollum. In the other Frodo and Sam were holding Gollum down and they had a sword to Gollum's throat. If they did not have power over Gollum, they were at least in a VERY advantageous position and I think that in that case pity would be a fitting thing for Bilbo and Frodo, given the situation. The same thing is true with Saruman. Frodo could have had him killed. If that is not power over a person, it is at least an advantageous position.