Alright, not random senseless violence, but senseless violence that he had once made some mention of, some time beforehand, in an incoherent note on an envelope.
Simply saying "i planned this" and having it noted, on an envelope, is not the same as "methodical" planning. How much planning does it take, anyway, to get to a public event at a mall, on time, in a taxi, and remember to have your gun and your ammunition ready, when to taxi picks you up, so that you can bring with you in the taxi?
An envelope with a few words on it is not the same as "methodical planning." "Methodical" planning requires at least several sheets of paper.