[Mulder]

Drugs or alcohol could have played a part and impaired his judgement, made him paranoid, angry, etc. Notice how it mentions his eyes being red:

Do you really think the cops would have let him walk if he was obviously drunk or on drugs when he shot the kid?

[Dr.Zoidberg]

"Notice how it mentions his eyes being red"

Just playing Devils advocate here, do you think there is any possibility the man had spent the night crying over the death, right or wrong? I'm not saying anything pro shooter or pro teen, but under the presented circumstances, it's reasonable to assume the shooter was badly traumatized by the death.

There are times when police officers in perfectly righteous shooting find themselves stricken by the knowledge of thier involvement in the death of a suspect. It may not be common, but it has been known to happen.

As to the rest, it's been said already but bears repeating, we are not privy to the facts of the matter and right now suppostion and airchair crime scene investigation is little more than mental masterbation. It may feel good, but in the end it causes a mess and upsets the spectators.