Who cares why he died, what'd he leave you in his will?
I think he's one of those NPCs who can die unexpectedly. It's either some buggy accident like from being rendered in the wrong place when you load the cell he's in (did you just enter the cell?), or it's scripted somewhere so you will get a random inheritance. I had that happen to me with an NPC who was just out of my sight for a second, and there was nothing visible that could have killed him. The key thing is something to do with him moving into the Arentino house after you kill Grelod, and Aventus moves back to the orphanage - neither of which happen. No idea about the beds, and I've never experienced this, except for the Whiterun stables that 3 NPCs live in and there's only 2 beds.