What is the function of 了 in the sentence "她会杀了你的"?
A small class of verbs (including 杀，喝，吃，扔，忘) can take 了 as a resultative complement meaning 掉. These verbs can use 了 in places where it might otherwise sound odd. In this case, 杀了 is not expressing completed action; rather, it expresses the result of the verb.

Some other examples:


  我不会忘了。 I won't forget.
  把它吃了！ Eat it!
  他把旧的衣服都扔了。He threw out all the old clothes.


Grammatically, 了 here functions as an indication of the verb's result. In the last case, it simultaneously expresses completed action (and, I suppose, new situation.)

EDIT:

Here is a more complete lists of verbs which can use 了 in this way (from the very useful 《现代汉语八百词》):

忘、丢、关、喝、吃、咽、吞、泼、洒、扔、放、涂、抹、擦、碰、砸、摔、磕、撞、踩、伤、杀、宰、切、冲、卖、还、毁