I probably missed something
Feel free to add if any important info was forgotten by me.

It's not Ronald ,because blood on his cloth was his , not victim's.
Not Walter , he was a drug dealer and their illegal job required at least 2 people to be there. They also made furniture , remember?
Not Emma , because her medical report stated that she was stable and 'fine', per say.

It is Anna Terner.
She had the motive to do this .
Explanation .
She knew the names of the victims when her Mother mentioned them .
Life of her Mother was miserable and in the fragments of an old case was stated that she was preparing 3 women for important roles, but those roles were take by victims,who was dericted a party.
Our protagonist was the past straw.( Maybe River was next, but that's only my suggestion.)
So it was a failure to some degree on hers side, although mostly by a party.
Obviousl injustice.

I think Anna wanted some justice for her mother's efforts that didn't achieved success.

Additional information from retired detective and our friend helps too.
Although I have to admit : It was hard.

so who is killer

Pretty sure as well, but damn, it was literally impossible to guess without watching the cutscenes during the first pick.