opatience: i'm a christian (and a jew, and that's a long story). anyway, biblically speaking "killing" is not wrong. "murder" is.
biblically, there is a time for everything -- war, peace, sowing, reaping, being born, dying. taken all together, it's my belief that "killing" has its own time as well.
in the torah/old testament, god HELPS his people (the jews) defeat their enemies and crush them on the battlefield -- in some pretty graphic ways.
in the christian bible/new testament, christ's way was considerably different. instead of the old eye for an eye, instead of sacrificing animals, the idea is that if someone strikes you, you are to "turn to him also the other cheek."
in the OT, things were different. killing had its time, its purpose, its reason for being. this was okay with god (and in some cases, it was commanded by him). in the NT, however, even the idea of doing harm to another was a sin.
that said, there's not a christian in the world who can or has or will ever live up to the ideals of christ. the problem is, there are some who realize this and do their best to learn from christ's examples, and there are those who seem to hold themselves above -- or at least apart from -- all of that.
there are some christians who are the very height of hypocritical. killing's "okay" if it's war, because "that's just different." (meaning "it's okay, because i personally think it is"). and besides, that's not MURDER (which is a sin). however, many of those same christians are 100% in support of capital punishment -- even thought that IS a form of murder: it is intentionally taking the life of a human being when that human being does not present a direct threat. it's killing, in other words, without being necessary to save one's own life; it's killing OUTSIDE of self-defense. i call that murder -- but then, i also have no personal problem with the death penalty in the USA for the most part. however, there are christians who will rail about the "sanctity of ALL life" (meaning only the unborn) but who would gladly flip the switch to kill someone on death row.
so this "sanctity of life" is, to some christians, merely a matter of picking and choosing based on PERSONAL opinion -- not based on anything god or christ ever ordained.