Killing or wanting to kill often represents a desire for power or control, often based in feelings of powerlessness. Killing someone can mean:
Killing someone in self defense can mean you're feeling attacked somehow (mentally, emotionally, physically) or that you need to defend yourself or stand up for yourself somehow.
Killing someone accidentally can mean you're afraid you'll accidentally hurt that person, that something you do may be detrimental to them somehow, or that your subconscious mind is thinking things through to make sure this doesn't happen.