The evidence (in 55+ exhibits + testimony):
Motive:
Erin seeing others, Erin breaking up with Chris, Chris admitting he had difficulty dealing with it, Chris admitting he killed her, Chris knowing that she was shot despite no credible method for gaining such knowledge.
Means:
Chris having Winchester rifle of consistent caliber that killed Erin. Chris having round saved in a Chinese puzzle box consistent with the murder bullet. Chris trying to explain it away that it was his "bullseye at the target range" (normal people would save the target). Police seeing rifle bolt in Chris's apartment and then it disappears when they come back (thus, making ballistics impossible as Chris would know). Chris throwing shoes that he had worn many times a week suddenly away (because there might be mineral samples on them).
Opportunity:
Chris being called for duty (for Greensburg tornado) and claiming that he was unavailable. Chris coming into Erin's home repeatedly when she wasn't home and her checking her house to seeif he was there. Chris seeing her dancing with another man that night at Central Station. Chris having no alibi besides "he was drinking" that night.
No credibility:
Chris fleeing the jurisdiction, STEALING another person's Humvee and attempting to escape multiple Colorado officers. Chris convicted of those crimes. Chris admitting that he killed Erin upon his capture. Chris telling how he waited and gun went off accidentally. Chris changing story at trial to never having been there against his attorney's advice.
Yeah, WHERE's THE EVIDENCE?
Pretty much all they were missing was the murder weapon . He destroyed the usefulness of that. Shooting from outside, he knew that it was unlikely that forensic evidence from inside the home could go against him, especially when he was sitting outside in storm conditions.