If you read the description of the accounts -- it's pretty complicated and drawn-out. Lots of write-down and i'm sure the robbers gave all kinds of fancy accounting of what happened too.
It's NOT the cops job to decide what happened or who is at fault, etc. It's the DA's.
The cops just followed protocol. Would YOU wanna get chewed out by your boss when you go back to the station and he asks "That guy, with an illegal gun, shot an unarmed person, point black -- you arrested him right?". "uhh. no Sargeant". "Why not?". "Uhh, cuz he was a nice guy?".
Fact is -- 99% chance the DA won't press charges against this guy. So relax.
The cops, under training, all they can do is write down everything that happened, make the mandatory arrest, then let the judge and DA figure it out.
That's how our justice system works. If you don't want cops becoming judge, jury and executioner -- then you must SUPPORT them when they go by the book and do their job.