My theory is that "killing = bad" is a default canned answer that society says is "correct" and people don't really think about what-if-I-am-faced-with-a-tricky-life-or-death-situation scenarios since these scenarios are not realistic to begin with.

I'm not very convinced that the assumption that people have a fixed moral system is correct. It doesn't make a whole lot of sense (evolutionarily speaking) to optimize for some abstract ideal consistency rather than fitting into a group. As far as living in a society goes, telling your social circle that you shun all forms of human killing is one thing, actually making a life-or-death decision when there are tricky caveats is completely different.