He was killed with a shotgun blast to the head. The official verdict was suicide, and shortly after his death, you had a lot of conspiracy theorists claiming he was murdered. That's pretty typical, and at first I just blew them off, but now I'm not so sure.
They point to the level of heroin in his blood, saying that if he took that, he would have been incapacitated, unable to pull the trigger. Also that there were no prints on the gun, and that the handwriting on the second part of the note didn't match the first. I saw a doctor on TV analyzing the autopsy, saying that the numbers were affected by the fact that he shot himself right after, therefore the heroin didn't have time to circulate through his body, so it was highly concentrated. There WERE prints on the gun, they just weren't clear enough after days in those conditions, and the handwriting is different because he was on drugs. Still, it is possible. Courtney Love definitely had a motive, since I read he was planning on divorcing her. One thing that no one talked about was that he was shot with his own gun, that he had purchased a week or two earlier. How would the killers know about this? I'm sure he didn't tell everyone he knew about it.
The bottom line is that he seemed like a really depressed guy, a drug addict. It seems completely reasonable that he killed himself. We'll never know for sure.