1. The Heroin
First of all, if the heroin had been injected post gunshot by the "murderer", it would not have entered his bloodstream, since the blood stops flowing after you die. So we know the heroin was injected pre gunshot.
Arguably he should have died before the gunshot due to the large amount of heroin, but if he had, the medical examiners would have been able to tell that the gunshot was post mortum and that he actually died of the overdose. We know that the gunshot is what killed him.
So regardless of whether or not he was murdered, we know that he was alive from the point of injection until the point of gunshot. What we do not know is how long this period was. My personal opinion is that it was a very short amount of time, just long enough for him to numb himself and pull the trigger. If he had waited much longer, the heroin may very well have caused his death instead.
So the point here is that the amount of heroin in his system did NOT cause his death. It is not really relevant WHY it didn't, perhaps he had an insanely high tolerance. People focus too much on the one piece that doesn't make perfect sense, and end up introducing far more theories that don't make sense, without introducing any real explanation of the heroin at all.
What is relevant is that somebody else murdering him would not make it any more likely for him to have been able to survive the dosage. The fact is that he did survive the dosage, and that suggests neither that he killed himself nor that somebody else did.
2. The Divorce
Actually, it was Courtney who was contemplating leaving Kurt, and not the other way around.
Recalling back to early 1994 before Live Through This even came out, Courtney Love was not nearly as well known as she was in the forthcoming years. She was not quite as out of control and Kurt really had no reason to leave her at all. I think Kurt's death pushed her to become a little more reckless and the rumours of her involvement pushed the public to lash out at her. But all of this happened AFTER his death, so none of it would have been a factor in their personal relationship.
There is the factor that Courtney Love and her band Hole did become much more well known following Kurt's death, and some people have suggested that this was one goal in the "murder". It's no secret that Courtney wanted to be famous, but in reality this is actually one of the factors that lead Courtney to consider leaving Kurt.
Kurt never wanted to be famous, or an icon. He was just a quiet artist who wanted to make great music, and the world noticed. I think he would have been much more content, and possibly still alive today, had the explosion of Nirvana into the mainstream not have been so great.
But Courtney did want to explode into the public eye, and she was becoming more and more frustrated with Kurt and the fact that he wanted to retreat more and more from the public eye. Also, Courtney was making a valiant effort to clean herself up and get off the drugs. Kurt had no intentions of cleaning up, and this was also difficult for Courtney to deal with.
After Kurt's death, it was no surprise to me that she fell right back into her old ways. This also suggests that she did not kill him, because obviously his death tore her up so much that it drove her back into the drugs. If she had been a part of the "murder", it would likely not have affected her so and she would not have returned to the drugs.
3. Conclusion
Anybody who thinks that Kurt Cobain did not kill himself is either a complete idiot or simply in denial.
There is absolutely no physical evidence whatsoever of a "murder", and the whole basis for controversy is the inexplicably high levels of heroin in his system. That and the fact that people simply don't want to believe that an icon like Kurt Cobain would take his own life.
In all of the ridiculous theories flying around, not a single one has given any explanation for why he survived the overdose. And that means that all theories, those of murder and those of suicide, all suffer from the same minor flaw. Face it folks, some things are just a mystery. Everything else considered, suicide is the only theory that make sense.
Anybody who knows anything about Kurt Cobain knows that he was destined to kill himself. All his life he suffered from depression and uncontrollable stomach pains, and found heroin to be the only thing to help. But his drug problems were beginning to ruin his marriage, his introverted nature was taking its toll on his career, and the pain of life was simply too much to take. Only a month earlier he had overdosed in Rome, but was rushed to the hospital by Courtney.
If Courtney had wanted him dead, don't you think she would have just let him die there on his own without becoming involved in a murder scheme? Do you really believe that Courtney would have her own husband and the father of her 1 year old child murdered simply to further her own career? Is this really more believable than the simple fact that Kurt Cobain wanted to die?
He bought a shotgun before being forced into rehab by Courtney. He broke out of rehab and locked himself up above the garage where nobody found him for days. He took an overdose of heroin, perhaps itself an attempt at suicide. And finally he shot himself, to ensure that this time he did in fact die.
There is absolutely no reason to believe that his death was not of his own choosing.