This logic quickly leads to the absurd.also i just noticed something, if you die because of a stage hazard, the stage then killed you. If it had not been there then it wouldn't have killed you.
If you recover to a ledge, the stage saved you. If the ledge wasn't there, you couldn't have recovered to it.
If you get killed by a smash attack, the stage killed you. If the ground wasn't there, then it wouldn't have killed you.
In terms of winning and losing, the only thing you really need to focus on is that when you lose, you did less effective things to win than your opponent. We can call a stage a bad stage when the options required to win are degenerate or when the stage removes the effects of the players' choices. When you start trying to go beyond that into "the stage killed me", you start looking at things as problems that are not and start favoring banning more than you should.
About PictoChat, given that that missile has fixed knockback and never kills by itself, yes dying after being hit by it would indeed be your fault.