Not sure what it's like in other countries, but here you'd probably get away with killing someone if you can prove your life was in immediate danger. This is the only time I could ever justify killing someone - if, personally, at that moment it was either you (or someone with you) or them.
This is entirely different from seeing someone in your yard and coming out guns a-blazing. Tony Martin* take note.
*Shot a teenager in the back and then claimed 'self defence'