The evil path it is. Although Dragon Age supports ethic relativism so I'm not sure evil applies here.
"We'll kill the guard," you say. "It is the only choice." Fern looks to you with stern disapproval.
"Very well," the blood mage says. "Bring me the cell keys."
You move to the upstairs and dispatch the guard quickly by drawing your sword across his throat from behind. You feel an immense sense of guilt for the cold-blooded murder, but it couldn't have been helped. You must be allowed your revenge. Nothing else matters.
You bring the keys into the dungeon and open the cell for the elven blood mage.
"Now open a door into the Fade," you say.
"With pleasure," the elf says. "After all, my word is my bond. Now, I'll just be requiring a sacrifice."
"But the guard," you say. "The guard was your sacrifice. I've just slit his throat."
"You misunderstand," the elf says. "The guard cannot be the sacrifice. I mentioned killing him to get the cell keys, nothing more. No, I'll need someone to drain right down here where we'll open the passageway."
You feel betrayed. This guard died for little more than a miscommunication, and you're barely closer to entering the Fade. What now? You've already killed one innocent. If you chose another path now, his life will have been taken in vain.
A) Travel upstairs to find another person to offer in sacrifice.
B) Change course and offer half the lifespan of the mysterious baby that Fern has been protecting.
C) Change course and offer half of Levina's lifespan, despite her disapproval.
D) Change course and offer half of your already short lifespan with the understanding that there is a good chance you will not be able to finish your quest by the end.
A does lead to something, so it's not a dead end. As others have pointed out, B & C are most likely shitstorms waiting to happen. D might be risky, who can say?