Most of the ones i can think of are either drug related or disease/failing health.(cancer/heart attacks etc.)
But i find this one quite...eery.
Jon Nödtveidt of the black metal band 'Dissection'.
Now, this guy was really a true satanist! He followed the 'Satanic Bible', performed rituals and so on and so forth. He had even spent 7 years in jail for helping in a murder. He committed suicide after performing a ritual, inside a circle of lit candles. And this wasn't because of some drug abuse or some brain altering substance; According to his beliefs, he had completed what he was supposed to in this world and there was no need of him living anymore. Apparently, he died with a smile on his face.
Weird sh!t
Not that i support what he did, but hey, he definitely made one of the most epic black metal albums of all time.