I killed Drelas who lives here, but I can't use his bed and such "Can't sleep while trespassing".

He's dead so what's he gonna do about it? Why can't I make this my home? it's much more cosy than Breezehome and already decorated and has an enchanter.

Ps3 so I can't mod it or console-command it.

I want to know the reason behind I am unable to make this my home? He's dead and I killed him because I wanted it. He is not essential nor is is house so there should be nothing stopping me!

Design decisions. There isn't anything you can do about it sadly.