Yeah there are several homeless encampments in that area of Durham (including near the Walmart that is up the road). But the ONLY scenario in which I see it being a homeless person is if someone from the Hawthorne woods saw KH leave at 4:30, and perhaps knowing those were 1-bedroom apts., figured the apt. was empty, and went to burglarize it. After lucking upon an unlocked door, encountered FH or surprised her in bed, and mayhem occurred. I don't imagine that's the case (and as you say, doesn't account for the note), but can't rule it out 100%.
What I do rule out is that this was a planned or premeditated murder (jmo) -- I still believe it was an assault/rape gone too far when FH resisted. If you're going to murder someone you take a knife/gun/rope/poison... along; you don't plan on there being a rum bottle sitting around.