To win this achievement, you must not kill anyone or do anything that may lead them to dying/killing later on. If your choice leads them to turning into a Skal or becoming a worse version of one then your choice resulted in death which may be the reason this isn't popping for some.
Below is what I did to get this achievement, if you copy this like for like then you'll have no issues.
Also, I know it's probably obvious but please remember not to kill any of the NPCs you talk to along the way. Killing the Vampire Hunters, Ekons and other attacking monsters is fine.