Smart question! You must notice that there are 2 language pieces in the sentence. 会will 了particle of action completion. Never confused about it.
As you know, there are 3 kinds of completed actions in English: Past perfect tense, present perfect tense, future perfect tense.
Then you can understand the sentence 我会杀了你的。means I will kill you./ I will have killed you.
I think it is the point of your question; 了is not only the meaning of the past but also action completed.
Enjoy your learning!