The first is Scottish crime writer Ian Rankin who interviewed Perry regarding her involvement in the murder and the second is Peter Graham who was said to be working on a book about the murder when he made the comments I've quoted.
And from Peter Graham via Second Sight:
Perry refused to be involved in [Graham's] book, but wrote Graham a letter last year mentioning the murder, which she referred to as "the tragedy".
"I thought this is a strange word to use when you've brained somebody to death and talk about it as 'a tragedy' as if somebody got run over by a train or something, " Graham says.
"But then when you think about it -- and I think this also came through in the film -- when she talks about this ghastly thing, she's really just seeing it as a tragedy to her. She's entirely seeing it through her own eyes, she doesn't consider at all what a tragedy it was for poor old Mrs Parker or Mr Parker or the rest of the Parkers, it's all about her.
"I think you do see that narcissism is still there. The mere fact that she wants to have this film crew in her house, following her around . . . is a rather sort of egotistical thing to do, and that was certainly what she was like as a child and as a teenager and she doesn't seem to have changed greatly."
Graham says Perry's tearful explanation in the documentary for what she calls "the thing that happened" appears staged.
"At the very end you get her breaking down in tears and talking about it, and really what she's saying is something that she's said before in numerous interviews: in effect 'it was all Pauline's fault, Pauline was suffering from bulimia and Pauline was threatening to kill herself and I honestly believed that if I didn't help her kill her mother then Pauline would kill herself and that would be on my conscience'."
(Now I am off to search for Mr. Graham's book.)