Complex sentence: Just before the execution, Coatl confesses to Father Bartolomé that he killed de Silva, but before Pedro can be freed, Catana stabs him with a knife to spare him the degradation of being hanged.
Split into simpler sentences: Just before the execution, Coatl confesses to Father Bartolomé that he killed de Silva. Before Pedro can be notified, Catana stabs him with a knife to spare him the degradation of being hanged.