Hi Leslie, 

The pictured insects do not damage grass. In fact, they prey upon some of the insects that damage lawns. You have observed a ground beetle (Family Carabidae), predatory insects that chase after prey on foot. They began burrowing in the lawn because the turf had already died back enough to expose the soil, creating suitable burrow locations. In other words, they are a symptom of the problem, not the cause. 

I cannot diagnose the cause of the turf dieback from these pictures alone. It's possible that it was not even an insect that was responsible for the dieback. Most turf issues in CO arise from improper irrigation. Consider contacting your local extension agent for a site visit. Even if you would only communicate virtually, I recommend waiting until more of the grass greens up. It is difficult to make any sort of diagnosis when the turf is still dormant. 

Let me know if you have further questions.