Reasons are always discernible, no one ever does anything for no reason. You are not supposed to think kindly of people you disagree with, they can be your enemies after all. You just always have to realize that they are people just like you, not some monsters. Respect is earned through doing different things , depending on the values of one you are trying to impress, and whose respect you want to acquire.
You are suggesting fighting with one hand tight behind your back, there is no reason to do so.
One would think so, but there are those who do things without legitimate reasons. Sometimes, people just do things. That is a life lesson that is very much worth learning. And I often have amicable relationships with those I disagree with. I am a liberal, but my girlfriend is conservative. I am a Catholic, yet my brother is an Atheist. In both situations, we get along just fine. We understand that we have differences in opinion and respect the other nonetheless. However, there is a line of what is and isn't acceptable.
Also, if you're saying that fighting without magic is fighting with one hand tied behind your back, then you obviously rely too heavily on magic. Surrender and running like hell are still options.
Blood magic adds no extra risk of demonic possession.
I may need to double check the lore, but I'm fairly certain that blood magic usually requires that you form a pact or communicate with some kind of demon in order to learn it.
But to what extent can you really pretend to value the lives of Bandit 137 or Coterie Thug 45? Is there really a line being crossed when someone chooses to steal their blood to power the next spell, as opposed to say, searing the very armor they wear to their skin with fire? Or, sending electricity to course through their body, causing them to release their bowels and die without any semblance of dignity? Am I really expressing in some way that the lives of the people I kill are no less important than mine by choosing to do the latter?
I'm not pretending to value any lives. If I was actively involved in the universe of Dragon Age, I would value every life. The line being crossed when it comes to blood magic is that is requires loss of life in order to use it. Even the act of learning blood magic shows an intent to use others' lives for your own power. It's different from setting someone on fire, shocking someone, or plunging a blade into their heart. These things are done in the moment in order to survive. Blood magic basically means that you have to kill in order to kill, as opposed to kill in order to survive.