Hi Boy and welcome.
I think he decided before they even got to the concert. This is only speculation but I was once a teen girl, so I can totally imagine that Emma's infatuation ended when the real deal walked in the door and they spent some actual time together. Ever met anyone in real life that you first met over the internet? OK, well, they are never the same as you imagine.
Emma probably lost interest immediately, started to shun him a bit and gravitated toward Mel. THREE is always a crowd - one person usually feels left out. By the way they acted at the concert, I think this happened prior to them arriving at the event.
I don't know if it's true, the rumor that Emma and Shrimm had post concert coitus or what, but I think Sam knew he was stuck with them on a LLLOONNNGG ride home and at the house. Maybe he had been working through his anger in the usual way by fantasizing about how much he'd like to...or how powerful he would be if... or famous???...and then there was a trigger and the fantasy became a reality. Who knows.
What I do wonder about is how are the details kept so tightly sealed in your small area? Why is there nothing more the police or DA will reveal? If a reporter calls, will they reveal more details previously unknown?
Is nobody asking?
I'm sure they are overwhelmed by the amount of forensic evidence they have to go though etc., I mean, computers, cameras, cell phones, social networking sites not to mention blood and all the other evidence in the hosue. But they seem to be very, very tight-lipped on giving more detail. Reason?
Edited to add: Will you go shake something down for us?