I don't quite agree OP. Blood Mages, in most cases, are worse than Templars. Specifically Blood Mages.
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Not every Mage is bad. But when a Mage is bad, they get super bad. They go Blood Mage.
Templars tend to be kinda-bad. An occasion of one being very good or very bad will happen, but less often than a Mage going super bad.
Thus the idea is that the Templars act kinda bad, but for the sake of keeping others from being exposed to the super badness that Mages can and have brought to the world.
I don't think that we're necessarily supposed to love or hate Templars or Mages, but simply pick a side when forced to by plot. Decide what you value more.
Pro-Circle Mages are about on par with Templars overall. The Templars are typically not about to eradicate mages all the time, but instead put them in the Circle and control their place in the world.
Blood Mages are about on par with what seems Red Templars will be. Erratic. Dangerous. But also we may understand why they go that route. Blood Mages, at least at first, do it in desperation for their lives and those close to them. Red Templars so far appear to be going that way out of desperation for their cause and focus on the 'anti-mage' part of Templar instead of 'containment and safety'. Both make a certain kind of sense, but only if you're selfish - which even the worst Dragon Age protagonists can't be at their core; they're forced to be heroes and self-sacrificing in some respect.
A Blood Mage that uses their own blood isn't necessarily bad though. They have that 'self-sacrificing' element that may just save everyone in the nick of time. This is why we can play one.
But a Blood Mage that sacrifices the blood of OTHERS, without their express permission and a solid logic behind doing so... that's about the worst that a person can be. Worse than any Templar. That was the depths of depravity that Tevinter fell to. For all we know, they collected in proper resistance and response to whatever ancient elves did to them... but then they held onto their power, their domination, their rage, and became a hellish story to tell for generations.
For the record, I am typically pro-Mage in confrontations, pro-reformed-Circle and abolishing of the Chantry for ideology, and accept Blood Magic to a degree without some automatic death sentence. Even Dragon Age 2 didn't deter me from that, because I view it in a certain context (like you know, Kirkwall is a crazy place with a thin Veil).