Most irrational bad guy ever?

(spoiler) What was the rationale behind the bad guys trying to and in many cases succeeding in murdering those police officers and FBI agents? Surely if they wanted to get away with the rape/murder of the couple, killing an entire police force is not the way to keep a low profile. They even had a couple of days to think about what strategy to come up with, possibly get a story together, wait for the police to prove their case against them rather than confessing to anything, yet the only thing they conceive is to shoot an FBI agent when she knocks on the door. Even if they had killed her, what would have happened next? Even more FBI agents would have coming knocking at the door? Then what? Kill them too?