I agree that if Gideon killed Nathan he did it because he feared that Eva life was in danger .
But could Monica some how be involved, maybe she didn't physically stab Nathan but could she have got Clancy to do it for her.
And Nathan would have opened the door thinking that who ever it was either Clancy Angus Gideon or Stanton was bringing the money he demanded.
I think Gideon's abuser Gideon would make his life destroy him but I don't think he physically kill them.
I think Vidal sr is somehow involved maybe not as the abuser but did he allow it to happen, was it someone who had been effected by Gideon's father and had lot money and somehow wanted revenge.
I am sure that Sylvia will have lots of twists and turns in entwined with you. It is a romance story and not a crime story after all.