Time for the devil's advocate to step in... OK, so, if they cannot prove it wasn't a suicide, logically, they can prove it was possible for him to kill himself with that gun. Along with his suicide note which features, possibly, his own handwrite (althought I never read or seen it, so...), his clear suicidal tendecies, a Cortney Love alibi (pretty sure she might have one), no signs of strugle, and the fact it is VERY difficult to shoot someone in the mouth... there's no reason to doubt it was a suicide.