Sigh...it ain't a theory. Here are some facts:
1) A guy claimed that, a week or so prior to Kurt's death, he was hired by Courtney to kill Kurt. After he came forward with this information, he himself was assassinated.
2) The heroin that was in Kurt's body when it was found was several times that of a lethal dose. Had he really shot that much into himself, he wouldn't even be capable of lifting a gun, much less firing one.
3) The supposed "suicide note" doesn't come across as a suicide note at all, but rather a note explaining why he's quitting music and divorcing his wife. The only time it comes across as a suicide note is the very last line, which is written in different handwriting than the rest of the note.
The main reason this was never investigated was because people were like you, and didn't fucking care about Kurt because he was just some guy who had problems with depression and drugs. It "makes sense" that he killed himself, and nobody wanted any other explanation.
Seriously, there's a great documentary about this called "Kurt and Courtney." And they must have had something right, because Courtney herself tried to bribe them from making the movie(much like Vince McMahon did with Wrestling With Shadows).
I didn't want to get so in-depth about this, because like I said, it bothers me that nobody seems capable of ever talking about Cobain unless they're talking about his death.