Solas was actually the one who finally convinced me that blood magic was not inherently evil. While he does not use it himself, he believes that there is nothing "evil" about it. People can be burned alive by fire magic, frozen to death by ice magic, fried by lightning magic, crushed by earth magic. What makes that magic any less "evil" than blood magic? Is a sword evil because it cuts you in half? It all comes down to the person using it.
Jowan- Dabbled in blood magic to save himself and avoid being made Tranquil, a fate most mages consider worse than death.
Merrill- Used it to try and help restore her people's history. She was naive and short-sighted, but she meant well, and was actually trying to do something to help the Dalish instead of sitting on her ass and letting things carry on like most Dalish.
Every mage you kill in Kirkwall- A mixture of psychopaths who abused their powers because they could, desperate people who feared the corrupt Templars of Kirkwall, a couple of Tevinter slavers/magisters.
Orsino- Pushed over the edge by the deaths of his charges and the strain of Meredith and the Templars and gave into temptation because he no longer cared about living or dying.
The Wardens- Gives them an edge, which they always need.
It all comes down to the individual. Dorian even says: what's the harm in using your own blood? The Chantry and the Dalish are just short-sighted and superstitious, and the Qunari hate magic in general.