Motivation of the murderer in Season 3

This Reddit post attempts to explain it:

From what I understood, Helle Anker used Freddie Holst's sperm to have a kid (Martin Anker) but destroyed all evidence at the clinic she had set up. She then she must've told Martin to look up the code at the clinic to find his real Dad and when he did he would've seen he had a brother (Emil) and that must've been how they met and I guess how Emil found out about his real parents.

So his 'creators' were his Dad and the woman responsible for setting up the clinic.

This Guardian commenter has a similar explanation:

For anyone wondering about the whole Emil/Morten relationship and why Helle Anker was referred to as Emil's "creator", I believe it stems from the fact that Helle Anker set up the fertility clinic. She used Freddie Holst's sperm for herself so that she could give birth to Morten, and later in life gave Morten some more information about his beginnings. As Helle ran the fertility clinic, she was able to give Morten the infamous code for Freddie Holst's sperm donation to Anne-Marie Larsson. This enabled Morten to find his half brother, Emil Larsson. This backfired spectacularly. Emil resents the fact he was ever born and takes revenge on Freddie (his sperm donor father) and Helle Anker ("his creator" i.e. The lady who ran the fertility clinic in which Emil was artificially inseminated). Emil was none the wiser about his beginnings until Morten contacted him.