There are many things Delta did to Comair after acquiring full ownership.
There are three events which lead me to believe Delta engineered the failure of Comair. While each event couldn't cause the failure of Comair by itself, these actions by Delta certainly made it more difficult to operate efficiently.
First, Delta took 15 CRJ-700s from Comair and gave them to ASA. This was during contract negotiations and Comair never saw those planes again.
Shortly before bankruptcy, former president Fred Butrell, aka Portfolio Phred, convinced the pilot group to accept a pay freeze and give up the B-fund contributions. One of the carrots dangled in front of the pilot group was the EMB-170. This agreement/LOA passed by six votes. I voted against it as I didn't see how a company soon headed to to bankruptcy could finance the purchase of these new airplanes. Once the LOA passed, Butrell went to Delta in an effort to get a EMB-170 program started. Delta told him those airplanes were going to Republic (hello, Shuttle America!) and Butrell resigned.
After bankruptcy, Freedom lost the -900s they operated for Delta and they were to go to Comair. I held one of the vacancy bids for that "promise." The transfer was cancelled and those airplanes went to Mesaba, now part of Endeavor.
If all those airplanes had remained at Comair, or been delivered as promised/implied, Comair would have been a more profitable operation and probably still flying today...IMO.