Just the fact that all Kurt's "friends" are so evasive to this day...
Should be proof that Kurt was murdered
At the time I can see why they were evasive - they were all druggies feeding off a crazy woman who was prone to violence. However if it was just about being paranoid junkies, the statute of limitations are long over for anything they could be hung up on legally for using and dealing. Being scared of Courtney might be a reason to stay silent in the short term, but eventually you stop being afraid.
The only reason they are still evasive is because they knew or assisted someone commit murder and then tried to cover it up - there is no statute of limitations for murder.
If they were ever convicted they know that the public would be so outraged (even people that weren't fans) that those who don't go to death row, will probably spend very lengthy sentences.