One man does not prove the case at all. The root cause is the answer. He never had a problem with murder. He never had a problem with demons or blood mages. Most people will not kill someone's child because their child is sick and dying.
Most people will not kill someone else's child because their own is sick and dying because in our world killing someone else's child very rarely will save your own sick and dying child. There is no correlation between the cause and the proposed solution.
With blood magic and demonology, however, sacrifice (of yourself or others) can save a child. Blood magic and demonology can do many things to preserve lives that no other known methods in Thedas can do.
The majority of the pop. are not murderers.
The majority of the populace will, however, kill other people in various circumstances with suitable pressures. People will kill others for an extremely wide variety of rationals ranging from abstract morality to selfless wellbeing of loved ones to personal interests. We call them a variety of things other than murderers, of course- soldiers and police are a common honored example- but they can be conditioned and willing to kill all the same.
All things considered, it's relatively easy to drive someone to kill someone else. There's a very long, sad, and successful history to attest to that. Actual pacifists are rare- it's really a testament of successful societies that a majority of the populace now doesn't even need to be able to kill, let alone willing to. With the exception to sexism, it used to be a common assumption that anyone could be called upon (drafted) to go kill someone else on behalf of someone else.
Compared to the relative ease of drafting an army for large-scale slaughter, believing that people would kill in the name of people they strongly care about is pretty simple.