Just a guess, but it looks like to me that the Perp invited the Vic to smoke some dope.
Vic misinterprets the invitation and is expecting a romantic interlude.
Perp freaks out when Vic whips it out. Vocal altercation morphs into physical altercation.
Perp stabs Vic. Decides to hide crime by dismembering victim.
Discovers after lengthy decapitation process that dismembering is not practical.
Covered in blood, having left lots of DNA evidence everywhere, he decides to turn himself in.
Bingo!
The problem with that scenario is that the perp was carrying a weapon capable of decapitating someone’s head. I’d say there was some premeditation there.