Complex sentence: She confesses to them she tried to get out of the deal with the Crossroads Demon by trading the Colt, but once she gave it up, the deal was changed so she had to kill Sam as well.
Split into simpler sentences: She confesses to them she tried to get out of the deal with the Crossroads Demon by trading the Colt. Once she gave it up, however, the deal changed so she had to kill Sam as well.