She does not summon demons, she uses blood magic to fuel common spells.
She uses use blood magic, while she is helping some mages to escape, while raiding a chateau, while helping Isabela to get new ship, ... basically while taking part in all the illegal activities Hawke conducts.
story/gameplay segregation
She doesn't have to use blood magic while helping Hawke, in a lot of my playthroughs she does just fine without it.
Granted she does use blood magic, story-wise, to get through the barrier on Sundermount, you can assume she always uses it in battle if you want.
Still not on anything like the level of a serial killer.
Story failed to sell me on the 'magic made him a killer' bit anyway. Blood mage or no - Quentin was insane, and no more dangerous, ultimately, then the insane non-mage who slaughtered all those elven children.