I made a response to a similar comment in another location (see ref 1).
I only want to respond to the following claims:
> he is not a trained EMT with access to a trauma kit that would stop 4 GSWs [gunshot wound, clarification added]
There are other cases where Minnesota police officers are documented to have responded to gunshot wounds (ref. 2) Indeed, certain MN police departments provide additional training, beyond basic first aid, for trauma situations - see the edit to ref. 1.
Further, you're implying that highly-specialized training ('EMT'), specific medical supplies ('trauma kit'), and injury severity ('4 GSWs') are important in considering rendering aid. My contention would be that, those factors absent, having a gun drawn and yelling at a victim are not appropriate responses to an event of this type.
> he is also alone and has to secure the scene until backup arrives
Backup is clearly present by the time the video is rolling and multiple officers appear on the video when Mr. Castile's girlfriend is removed from the car.
[1] https://news.ycombinator.com/edit?id=12047969
[2] http://www.startribune.com/documents-detail-accidental-eden-...