I think your missing the key element here and that is your own humanity. Sure people can tell you its "ok" to kill, but is it really? Should we as human beings adopt the thought process of "Its no big deal" or "think nothing of it" as a general way of thinking? That seems to be what Dictators around the world have done, and look what it has lead to? Genocide? Mass graves hidden around their country, innocent people killed to make a political statement?
Losing touch with your humanity makes you no better than the person trying to kill you, you essentially become the monster. It is NEVER "ok" to kill someone, does this mean you shouldn't? It depends upon the situation obviously, if your faced with no other options then to either lose your life or take someone else's then by all means, do what you have to do. But to dismiss the value of a human life so non-chalantly is not only disturbing but indecent.