Thoughts and speculations before series finale (spoilers)
Spoilers ahead ...
1. The killer knows how to deliver a baby. Not something anyone could do. Gustav, who is seen working at the hospital, is the only one who could potentially have the knowledge to assist at birth based on what we know. Then again I think he has an alibi for Jeanette's abduction.
2. The last crime scene where Annika was supposed to be killed was a recreation of the basement room in the foster home. All five children told the police that room was locked and they never went there. So the killer is someone who knows the room from the inside. Someone who lived in it, probably. When the foster couple was killed and they discovered the room, Henrik said that it looked like someone had lived there. It can't be a coincidence that this keeps resurfacing.
3. It could be that Freddie is the father of more than one character. He could be the father of Emil and Rasmus or of all the foster children. Remember that Annika's father is unknown - maybe it's Freddie. This would also be great irony: Freddie spent years yearning for a child while five of his children were growing up without a father.
4. I think the killer is/are one of the following:
Rasmus
Emil and Rasmus
All the foster children working together
5. To me Rasmus is more suspect than Emil, but I can't write off Emil for these reasons:
- I think his injuries earlier in the series were too light, possibly self-inflicted
- his version of childhood events differs from Gustav's in important details that give Annika a motive for revenge.
6. It's also interesting that the killer has been getting "sloppy" (Jeanette and Annika were found before he could kill them) ever since Rasmus started doing field work on the case. Not enough time to finish the job?