10 - Is it a sin to kill? Should Christians be Pacifists?

Is it always a sin to kill? Should all Christians be Pacifists by nature? Does scriptures position on violence change from the old testament to the new? If war is always bad then why did God ordain it in some circumstances? If we tell our children that hitting is always wrong and we should "turn the other cheek", why did Christ chase the money changers out of the temple with a whip? Why does the book of Revelations say Jesus will return as a conqueror? All this and more.