It definitely baffles me too. I think it might also be a large part of what makes this case seem so disturbing to me. The calm, nonchalant, and bored demeanor of this person is so creepy!
I've read a lot of material about self-defense---because I tend to be an overly paranoid person (probably because I spend too much time reading about horrifying crimes on WS
---and everything I've read talks about the importance of recognizing signs of a fight-or-flight response. Even seasoned criminals and true socio/psychopaths display these physical signs right before an act of violence because there will always be some risk involved. The outward appearance of the fight-or-flight response is not something that can be controlled or hidden. That's why experts tell us to pay attention to anyone acting unusual--pacing back and forth, darting eyes, jittery or jumpy, can't sit still, an immediate change to the tone, speed, or pitch of voice, sweating a lot, etc.
I guess it would be difficult to discern most of those things from video footage though. I've also read that sometimes, especially with an experienced criminal, this process happens rather quickly---within minutes, or sometimes even seconds, before they attack. It still seems strange and freaks me out though.
IMO, the suspect's gait and mannerisms look similar to those of someone cognitively impaired.