It's notoriously difficult to asses whether someone is guilty or not from their demeamour
Look at it this way - assume for the sake of argument she's innocent -
she's suffered a major trauma because she's had her mother in law who she says she was close to and two friends die in quick succession. That's traumatic enough.
Add to that it was after a lunch she cooked - so she's got the personal overlay of 'did I do something wrong here?' and that trauma.
Then she's got the whole world thinking she's an Agatha Christie villian cooking up death for her friends and family. So she probably thinks (Even if she is innocent) that she needs to "act" innocent and this is important for her legally and socially.
Then on top of all that, she comes home to be doorstepped by the media which if you've never had this happen to you or see it close quarters is really intrusive and shocking. the cameras are in your face and your mind is a whirl about the appropriate way to look, whether you should answer etc.
Pack all that together and most non-media/non-celeb people who are not used to dealing with the media are going to come across out of character and possibly very strangely.
It's very harsh to write her off as guilty based on that video.