It's a little disturbing.... scratch that, it's really disturbing hearing about this story regarding the bus stabbing in Manitoba.
It's mystifying trying to comprehend what the person was thinking. Based on observations from the interviews, it seems like this was a random killing. A random attack, on a random person (unless he really, really wanted that corner seat at the back). It was premeditated though as the killer was armed with a survival knife and described as calm during the slaying.
The neck stabbing is one stage of trying to comprehend this. But then, the killer goes back and decapitates the victim. To behead is another level of extreme instability and aggression. It doesn't really stop there though, as the killer ramps it up again and is a sociopath by trying to distress other people as well by carrying the head around and slamming it against the door.
If the rumors about cannibalism are true, then this just goes beyond gruesome. This is pure terror.