Would Have Killed Meaning?

Saying "I would have killed on/at something" is a colloquial way to say "I would have been really good at something".

Could have killed Meaning?

(somthing) could have (done something)

In the example: You could have killed someone! The meaning is that, although you didn't kill anyone, there was a high chance of accidentally killing someone. One common use of "could have ___" is to express how dangerous something was that happened.

Would kill for something meaning?

From Longman Dictionary of Contemporary Englishwould/could kill for somethingwould/could kill for something (also would kill to do something) to want something so much that you will do almost anything to get it or do it I could kill for a smoke right now.