>>500177651It's power. Power is not inherently evil. What matters is what you do with it. This is said pretty plainly every time it comes up.
However, that being said, the track record for blood magic is really, really bad. In other words, people typically do shitty things when they get that much power quickly. If normal magic is a glock19, blood magic is being given an assault rifle, and most people don't have assault rifle training.
Normal magic can shoot elements to kill people, but it can also mend and heal. What do we see blood mages doing throughout the games? Heinous shit like dominating people's minds and killing entire rooms of people at once. Can normal magic do these things too? Sure. But normal magic requires great effort and learning and preparation. Discipline. The firearm analogy works again. Why train accuracy and discipline with a handgun for years when you can just pick up an assault rifle and blast a room of people. That's part of the issue with blood magic.
There is a second issue too though. Using someone else's blood to fuel your spells is pretty evil. It's one thing to choose to make the sacrifice yourself, it's your body and your soul. But the second you decide you want more power and so you reach for the person next to you, you are falling down a slippery slope. Which I guess is one of the main issues too. It's a slippery slope.
Merril being a blood mage who wasn't in it for the wrong reasons, she wasn't a bad person, but it still giving her issues and tempting her is supposed to convey this message. We have a character in the series who is a good kid but was dancing at the top of the slippery slope.
Then we have characters like Jowan who ended up being shitty, and of course the tevinter mages who use it constantly for bad things, as well as the original cause of the blights.
