Why did so many of the headshot-victims survived?

Only in Quake and Hollywood you do! :)

Of course it depends on where in the head the bullet hits, just a month or so ago there was an attempted double homicide in stockholm where two people were shot in the head from a close range, and both survived.

In the case of Lasermannen, I think one reason so many of his victims survived was that he had modified his weapons to make them easier to conceal, and probably damaged them in the process. He had sawed off the barrel on the rifle he used, and had attempted to fit a silencer on his revolver. I remember in newspaper articles around the release of this mini series that they mentioned that revolver's can't be silenced that way (I can't verify if that's true), but also that the way he had fitted the silencer made the bullets he fired spin in a way that lessened their kinetic energy as well as accuracy, and didn't penetrate very deep when they hit.