The DA decides whether to file charges based on the evidence he has. If that evidence (physical evidence, witness statements, etc.) shows that the provisions of CRS 18-1-704.5 (2) are met, then he can't (and shouldn't) file charges.

In this case (there's more detail in the linked Reporter-Herald article), it looks like the statutory conditions were probably met:
* unlawful entry by Koontz
* reasonable belief that Koontz was going to commit a crime
* reasonable belief that Koontz might use physical force against occupant

Personally, I like the fact that DAs are prohibited from filing charges (or, if they do, a judge can dismiss them pre-trial). If a couple of thugs break into my house in the middle of the night and I shoot them, I should not have to spend tens or hundreds of thousands of dollars to defend myself from a criminal prosecution or civil suit.