It always seemed to me that loving my enemy was one thing; allowing him to hurt my wife or kids was another, but I wanted a Bible based answer, not just my own selfish interest.
Fortunately, I'm not the first one to ask this question, and I found a great answer at Biblical Self Defense
You should go read the whole entry, because he does an excellent job of backing up every point he makes with Scripture and he takes the time to provide the context of his quotes. In essence, his search shows that there is a Biblical duty to defend the weak and innocent from the strong, and that killing in the defense of others is not murder. On the other hand, killing to defend property might be. Going even deeper, he notes that even a justifiable killing can affect your standing in the eyes of God. You won't be punished for doing what you had to do, but there are avenues and ministries which are closed to the defender/warrior.
There's also a price to pay. We're all familiar with Matt 26:52 which tells us that all who take up the sword will die by the sword. This warning is echoed in Revelation, but there's some translation difficulties.
Rev 13:10 is translated like this in the King James Version:
He that leadeth into captivity shall go into captivity: he that killeth with the sword must be killed with the sword. Here is the patience and the faith of the saints.
But like this in the ESV:
If anyone is to be taken captive,
to captivity he goes;
if anyone is to be slain with the sword,
with the sword must he be slain.
Here is a call for the endurance and faith of the saints.
There's quite a difference between the two. The context is the rise of the first beast, who will be given power over all the nations and tribes, and will make war on the saints and defeat them. The ESV version, which is more literal, suggests that if the saints are destined for capture or death, then they will be captured or killed and must therefore endure with patience and faith. The KJV version suggests that the saints shouldn't even fight, but depend on the power of God to reverse their defeat.
In either case, we have to remember that death is not a bad thing if you are saved. Heaven still waits.
So killing another man, even in self defense or defense of another, is a very serious thing according to the Bible (of course, I knew that already) but it is not automatically the wrong thing to do. There are times and circumstances where it is both necessary and right to do so.
Like I said, go read it for yourself.