I also believe there is one true sin: being disrespectful of the gift of life. To disregard its protection and safety is insupportable. To fail in its protection and safety is a shame. To cast away life as if it's unworthy of protection and safety is unforgivable.
I believe there is a strong distinction between killing and murder. It's the distinction between what a parent does when protecting a child, versus what a criminal does when greedily taking everything another has. I believe that being forced to halt a murder via any means necessary (including killing) is not a crime, either in the eyes of man or God. YMMV, on that one.However if one is forced to do so does this mean they will be commended to hell,for the act of murder?
For myself, I won't allow my life or the lives of my loved ones to be taken without a fight, without a serious attempt to protect the mighty gift (life). That would be a sin, and that is something I will not do.
It's one of the tougher questions. Check with the "Robes" (priest, pastor, father) on that one.