The shooting was no more cold blooded than what all the others have done so far. Been thinking about that lately. In a way he was following orders. Walt told him just prior to the shooting that NO ONE MUST EVER KNOW ABOUT THIS ROBBERY. Misinterpreting his words..seems he was just stepping up, whereas everyone else just froze when confronted with the witness and his opportunity to flee.
Off topic....but what do you think Walt was thinking while referencing Victor flying too close to the flame ..getting his throat cut. Was it a veiled threat to Jesse? Mike?
It was a bit of a shock, nevertheless - that killing (and the introduction of the boy victim) didn't have to be in the story. OTOH to provides impetus for the story to move forward - Mike and Jesse deciding to leave, etc. Also makes the all darker characters than they already were - perhaps pressaging an untimely end for all of them. Is the spider to be their downfall?
Too close to the flame? I didn't see it as a warning of anything Walt might do, but advice to be careful (of danger from others).