Both brothers appear to have been assholes that were able to pre-meditate this plan over the course of months. I doubt they were balls out crazy based on that. After reading some of the note they both seem pretty narcissistic as noted by the comments about the family not being able to go on without them so they would just kill em all. My take is they were afraid that little sis, that had a full ride to NYU (?) , was going to make them look like a couple of assholes if she went on to be productive so they needed a way to assuage their embarrassment. I also saw a Beto sign in the front yard on one of the clips too.