This is just the way I see it.
All that she basically says, while holding a picture of her, Peter, and Harry, is "Oh, Peter. All the death you brought into our lives...".
My first thought wasn't, "Oh my God! What bad writing, it's so out of character for Liz to say that!". Of course she knows that Norman Osborn was insane and that it caused her husband to follow just the same path. But she DIDN'T know who Spider-Man was until...well, right then and there, heh. And she found out that Spider-Man was Peter Parker, Harry's best friend.
Yeah, she's going to feel a little upset about it all. Pete could have stopped being Spider-Man, I'm sure she's thought that. Because in her place I'd think the same thing. How come he just didn't stop being Spider-Man? Why did he have to keep continuing and just stop and be Peter Parker, Harry's friend.
As readers we're supposed to probably feel a certain way and say, "Well, it's NOT Spider-Man's fault!". I'm guessing we'll probably get a little more in depth with this later in this arc. I don't think we'll be leaving it like this with Liz Allan, that is unless Molten Man offs her.