A Time to Kill?

[Raynor]

Originally Posted by Elmo

Why did the Valar let the sons of Feanor live when they stole the silmarils after the war of wrath?

Well, it wasn't the valar who let them live but Eonwe, a Maia. I would say that the best explanation is that he had some foreknowledge of the fate of the silmarils and that the acts of Feanor's son were part of a larger plan. Also, the Valar granted pardon to the Noldor, subsequent to Earendil's plea on behalf of both Men and Elves, which can further explain why more pity was shown to these two than to the mariners. Furthermore, Eonwe stopped an attack on them because it would have only perpetuated violence. I believe that what Manwe said in the context of Finwe and Miriel can be applied here too:

Originally Posted by Of the severance of marriage, Later Quenta Silmarillion, HoME X

Neither must ye forget that in Arda Marred Justice is not Healing. Healing cometh only by suffering and patience, and maketh no demand, not even for Justice. Justice worketh only within the bonds of things as they are, accepting the marring of Arda, and therefore though Justice is itself good and desireth no further evil, it can but perpetuate the evil that was, and doth not prevent it from the bearing of fruit in sorrow.

[Legate of Amon Lanc]

Originally Posted by Raynor

Also, the Valar granted pardon to the Noldor, subsequent to Earendil's plea on behalf of both Men and Elves, which can further explain why more pity was shown to these two than to the mariners.

Just to complete this information, the Doom of Mandos was still at work when the mariners were sailing. You can see what Ulmo Himself says to Tuor in similar circumstances:

Originally Posted by Unfinished Tales, Of Tuor and his coming to Gondolin

And as Ulmo said these things the mutter of the storm rose to a great cry, and the wind mounted, and the sky grew black; and the mantle of the Lord of Waters streamed out like a flying cloud. "Go now," said Ulmo, "lest the Sea devour thee! For Ossë obeys the will of Mandos, and he is wroth, being a servant of the Doom."