This is, obviously, more of a question for law enforcement, lawyers but there are some very knowledgeable people who might be able to shed more light on the whole process of arrest and how the District Attorney decides to charge.
I've never been arrested and other than the occasional speeding/parking ticket I've never been charged with anything either.
From my understanding an officer of the law is called to a scene and determines whether or not a crime has taken place and arrests an individual for that crime depending on his interpretation of the circumstances, the laws that apply and procedure.
He takes the individual into custody and files a report with the suspected charges. The district attorney (DA) reviews those suspected charges, reads (hopefully) the police report and determines whether or not to actually charge the individual with the suspected charges.
I have heard of cases where the individual in question has NEVER even spoken to or seen the DA and charges are filed without so much as a single word being spoken in regards to guilt or innocence.
We also know there are those instances where officers will throw every offense and the kitchen sink at an individual to get him/her to get the charges maximized so that an individual will plead down to something that will stick.
All that being said... (phew)
There are a number of people who strongly believe that when officers arrive on the scene of a self defense shooting the only thing that should be said is, "I want an attorney."
It gives officers no choice but to arrest the individual for what may very well look like murder and then railroad them through the system on to an arraignment and trial date (though, hopefully, it would never come to that and charges would be dropped somewhere along the way).
If the last standing individual on the scene of the crime gives the officers nothing with which to work with they are left to assume the worst and write up their reports only as they interpret them.
How much influence do those reports hold with the District Attorney?
If a police officer has no other choice but to suspect something horrible happened because he can't get any information out of the individual involved and he writes up his opinion that a murder took place vs self defense, is that worse or indifferent to the district attorney?