Whydunnit, eh? Hm...keeping it lighthearted is definitely a challenge. I think some kind of blackmail revenge is the easiest option (security guards have power over others, and may therefore abuse that power for personal gain), but it could be something else. Of course, you could always do a subversion by setting up all these reasons that someone might have killed him, but it turns out that he was just in the wrong place at the wrong time (perhaps he caught someone in the act?).
The idea of it being a 2003-ish time capsule is really interesting! I always love seeing parody brands in media (especially parody McDonalds for some reason), but at the same time, it might be more effective as a time capsule if you use existing companies. Bit of a headscratcher, that.
Oh wow, I didn't know you could actually add visuals to Twine projects! I think a map might help clarify things, but then, you have murder mystery novels that get by without maps, so I think both ways could definitely work.
Definitely a neat premise, so I look forward to seeing how it goes! ^^