From what I understand, the guy had a thing with claustrophilia. Note: Not claustrophobia - the fear of confined spaces, but instead the preference and enjoyment of confinement and tight spaces. He had been regularly checking related websites on his computer. They did some actual experimentation and determined that it would have been possible to actually zip himself up in the bag and still have have the ability to grasp the lock from inside, through the bag fabric and lock it. There was no sign of forced entry into the place. No sign of other violence. No prints. Etc.
On the surface it LOOKS outrageously suspicious, but the actual most likely explanation - as bizzare as that is - is that he did it himself as a fetish thing. I presume he then wasn't getting enough breatheable air in the bag, passed out, and then simply died of asphyxiation.