When he went to the DEA office to lure Gus to Casa Tranquila, all of the Salamancas were dead at that point. Hector had nothing to lose by actually telling the DEA about his crimes, and Gus's, as he would have been returned to Casa Tranquila anyway. And even if his attempt to kill Gus was unseccesful, Gus would still have been brought down (kind of like an extra layer of "insurance" in Hector's plan). Was it just because of his hatred of Hank and the DEA? Or is there something I'm missing?