Unfortunately one would need to attain the supernatural power of seeing people's previous lives in order to see the exact working of kamma. Otherwise we should heed the Buddha's advice in AN 4.77:
There are these four unconjecturables that are not to be conjectured about, that would bring madness & vexation to anyone who conjectured about them. Which four?
The Buddha-range of the Buddhas, the jhana-range of a person in jhana, the precise working out of the results of kamma, and conjecture about [the origin, etc., of] the world.
That being said, the consequences to the kamma of killing could be multi-faceted and quite severe for that act would bring not only pain and suffering to the victim but also to the people around him/her. So by killing a man, beside the apparent suffering one brings upon that man, one also generates tremendous suffering to the man's mother who just lost a son, to his wife who just lost a husband, to his children who just lost a father, and many of his relatives and loved ones. So it's kinda like when one owes a huge debt to so many people, it won't be easy to pay off this debt in one nice and clean lump sum payment..