That's an interesting point of view, very white knight-like. Tell me, have you ever been on a real battlefield? Or ever in a situation where the only way to survive is to kill whatever moves towards you? You wouldn't say all that if you did. I myself have never been in a situation like that but I know for sure that I won't have the leisure to think about life and it's value, and the importance of the life of what's in front of me, if I ever fall into such a situation.
If someone is my enemy, enemy in such a way that I know their intent is to kill me(unlike a rival , or someone in your school or workplace bearing a grudge on you), then yes, it IS "okay" to take their life. Sure, I would try to avoid killing them if I am such a skilled fighter as to be able to do just enough damage to incapacitate them, rather than kill them. You may carry ideals such as "every life is precious" and whatnot but don't expect the enemy will too. Mercy may be something you're willing to show but don't expect the same from your enemy.
The third paragraph of your post is really what made me reply to you. "It encourages the idea that your life is more precious than your enemy's" You don't need Blood Magic to tell you that your life is more important. Your mind already instinctively tells you that whenever you fall into a life threatening situation, and you know what, your mind is bloody right. Seriously, if you don't consider your life to be more important than your enemy's, if you constantly focus on the enemy's well being while he slashes away at you, trust me when I say that you won't have a chance in a real fight.
Having said all that, I understand where you're coming from. I sometimes feel that way too when people on the forums say things like "I don't like that guy, I'll have to kill him soon", but that's when I come back to reality telling myself that it's just a game. Be happy you don't have to even think of such things in real life.
Fortunately, I have never been on a battlefield, nor do I ever intend to be. I've been in more than my fair share of fights, but I don't consider those anything more than schoolyard squabbles. I grew up in a town that is very conservative and pro-military. Many of the people I grew up with and am still friends with currently serve in America's armed forces. Some have killed and some haven't. I also have an uncle who was a member of the United States Secret Service for several years before joining the FBI. He is now retired due to extensive nerve damage caused by a car accident. My philosophy is based on their views as well as tidbits taken from Sun Tzu's The Art of War. If you've never read it, I highly recommend it.
The fact is, every life is precious regardless of who it belongs to. Each person is composed of a lifetime of memories, relationships, and growth that you may not have been privy to. When you are being attacked, it is kill or be killed. You aren't left with any other choice, but that doesn't make it okay. If you recognize that, you'll realize my view is nothing like that of a "white knight." Certain situations will call on you to do terrible things, but knowing the weight of your actions will always ensure that you take them with justification. It's the difference between killing a man for robbing you and killing a man who was trying to kill you. In one instance, the man is likely desperate for cash and has no interest in taking your life -- killing him is a gross overreaction. In the other instance, the man is trying to end your life -- killing him is necessary for survival.
As for the blood magic part, I think many of your statements are based on the misconception that I would not kill my enemy if attacked. If my enemy were truly trying to kill me, I would not hold back, nor would I let him leave alive. However, I would not take pride in my actions despite having proper justification. My actions would have just robbed someone of his or her remaining time in this world and as well as having taken away another's friend, relative, or spouse. My issue with blood magic lies in it taking power from the lives of others. The thought that, "I need more power and you are all nothing but fuel," disgusts me. If the question is, "When is Blood Magic justified?" The answer is, "There is never proper justification for it," especially when lyrium can also serve to power spells.
And I appreciate your last paragraph. I'm glad you can understand. I know we are only talking about a game, but people often act in the way they think they would in real life. I pray that I never have to see a battlefield. However, understanding the value of life is the one thing I can recommend to all people. With the way the world is today, it's far too easy to view other people as obstacles in your path instead of other human beings. We only ever see glimpses of others' lives and we treat them accordingly, despite the fact that they may just be having bad day. Perspective is key.
For some reason, I find that very chilling.
You should.