I agree that sandbagging in tournaments is wrong, but sandbagging out of tournaments isnt necessarily, especially against someone who isnt really in the smash community.
For example, one of my best friends is this girl who plays Jigglypuff. She can ONLY play when she drives from PA to come visit me, so to say the least, she isnt that great. I could probably 4 stock her on a relatively regular basis (im actually giving her my gamecube so she can practice more). Are you telling me, i should give her my all in every single friendly we play? I dont, I generally 2 stock her. I use the opportunity to do more risky moves in order to practice them thus making them less risky to use in tournaments, and i use it to practice some of my low tier alts.
I dont think sandbagging is bad, but the thing that you suggested isnt sandbagging, its being an A$$, The thing that is so annoying about what he did isnt that he was sandbagging you, but that he was taunting you, and making fun of you. If he was 4 stocking you with his best character and telling you how much you sucked at the same time, im sure it would be equally as annoying.
so your hatred shouldnt be directed towards sandbagging, it should be directed towards taunting.