That is certainly one scenario. But I've known a few girls around her age, troubled, and more than fantasizing about a suicide pact or being killed because they want to die. I think it's just a youthful fantasy for most, but kids in that state of mind can make terrible decisions. To me it's just as likely it was a pact and he didn't anticipate the real fall out from society and internally.
I doubt he ever even thought about the consequences. I also doubt we ever know with any degree of certainty. I certainly can't say you are wrong, just that it's not the only possibility.