If you kill an NPC, can you take over his home, as in, use his bed and store your things there. I really want one of the canol homes in Riften but they aren't for sale. I killed the guy living there but I still couldn't sleep there etc. Will this go change if I wait a while?

From my experience, even if you kill all the residents of a house, it's still considered their property judging by how it's still considered stealing if you take stuff. So prolly not.

The entire house will be set to his ownership regardless, so sleeping there with the well rested bonus and storing items safely will not be possible. You could try setownership command in console but that still may not work properly.

Just marry some poor girl (or guy) with a house and 'divorce' them. You didn't hear it from me.

Sweet, do you get their house?