I think he's guilty. One, he bought a knife. Why buy one when you aren't gonna use it? Secondly, emotional distress, huh? Well can that cause you to forget how to use a flick knife that you so handedly know how the same way it made you forget the names of the movies he watched? Thirdly, the murder was premeditated, no? Meaning, he had that knife open already before he went to his dad and plunged it in. A quick stab (normal flick knife holding position) would be possible if it was done out of desperation but he had planned it, meaning he opened it and changed holding position to create the angle that he did. We can all agree the old lady SAW someone but who? That is another question BUT we have to remember, if it were anyone else, what motive? I'd set him to the chair. 

Now what say you?