It doesn't matter if Kurt killed himself or not . . . but most real evidence does lead to suicide. Some of the evidence even suggests that a killing would be impossible, considering the time of death, angle of the gun, blah blah blah. But it doesn't matter - he's dead, no matter what, and he did hate his life, so maybe he's better off now.
Nirvana wasn't the best band ever and won't be. I love them, but I can still say that because it will be true. In the sixties and seventies, there were bands whose songs matched the feelings the people of the time had. Then, in the eighties, it was all crap, glam rock and nothing substantial or concrete. No feeling. So, when Nirvana popped up, it was the first taste of truth, of feeling, that everyone could get, that was available to everyone easily. That is why people licked it all up so quickly. Nirvana was a fantastic band, but many other bands have said more than Nirvana did, been more unique than Nirvana was (Cobain was original, but he was also good at borrowing other people's ideas), and continue to make music and/or have made more than five albums.
Bottom line - Nirvana was great, but it was mostly great because it wasn't underground. There are a lot of bands people in the mainstream have never heard of that are better than Nirvana ever could have been.