There's is no doubt at all that Kurt killed himself. There is not a shred of evidence to suggest it was anything but suicide.
All that stuff about there being too much heroin in his body to lift the gun is nonsense! Everyone reacts differently to a certain dosage of heroin. It is perfectly possible for an addict (like Kurt was) to function normally when full to bursting with the drug. And there were NOT two sets of handwriting on the suicide note!
Finally, the reason there are all these conspiracy theories is the same reason there are conspiracy theories about Princess Di and Bruce Lee's death - he/she was such an influential figure that people can't cope when he/she simply dies. Thus, they create a huge story behind the death because an 'ordinary death' isn't good enough for such an important person. Case closed!