THE INNOCENCE

Mark a writer is talking to his therapist stating how his sister. lizaa got murdered in his own house and how depressing this is.
his sister was stabbed 9times near her heart and how his heart breaks to even state this thing about his beloved sister. The therapist while listening to all is in awe as he finds that Mark is surely hiding something while narrating the story of the room which makes him uncomfortable and confused as he knew that how much mark loved his sister.
Everytime mark would talk about her he wouldn't stop exaggerating about how nice and polite she is but still he is trying to hide something is disturbing

so what is it which Mark is hiding who is trying to protect when his sister was brutally murdered?
who is the murderer? why did he murder such a nice lady??