[akhtene]

For one thing, perhaps Luthien (elf) and Huan (dog, even though born in Valinor) weren't able to kill Sauron (maia). And to deprive him of his body and let the evil spirit wander around - what good would it do?
Instead Luthien demanded that Sauron yield to her the mastery of his tower where Beren and other prisoners were kept. Otherwise, how would she hope to overpower the garrison of the tower, especially with Sauron's spirit around?