There was a profiler on the radio this afternoon who espoused the exact opposite theory, that younger brother was the main cog and brains behind it and basically he allowed/sent big bro to sacrifice himself to aid in his own escape.
Apparently there were reports he "drove over" his big bro (who may have been still alive) whilst escaping. I'm curious to see how his defense plays out. Will he play the "I was only the kid brother who was cajoled by bully elder brother" card? Seeing as he was hiding in a boat, he seems like the type who will do literally anything to save his own ass.