So what you are saying is that blood magic isn't merely about the power of your spells, but rather they can also enable you to perform unique acts?
And using it against enemies is a moot point. Armed combatants coming to kill you aren't victims and falls more into the self-defense we talked about.
Blood magic has two meanings. First, it is a method of using blood as a power source to augment mana, fulfilling the same role as lyrium but with greater potency (the extent of this has not been explained in detail thus far). Second, it is a means of casting unique spells that typically relate to the control of blood, mental domination, restoring the health of the caster by draining the life of another, sundering the Veil, and summoning demons.
According to her explanation Merrill was using blood magic in an attempt to purify the Eluvian because she did not have the "piles of lyrium" needed for her rituals... She needed more power than she could summon naturally, thus she needed to learn blood magic. She did not require living sacrifices because her own blood provided enough power to serve her purposes....
As for harming others in combat by draining them, I'm not really arguing that point... I think that "living sacrifices" for blood magic rituals is typically mentioned in reference to Tevinters sacrificing slaves to summon demons or enter the Fade. My original point was that Merrill wasn't involved in anything like this, so her magic would not have required blood sacrifices....