I'm curious... Doesn't it depend on how he's killed? If you cut his throat for example, there's a huge blood loss, so I'd imagine the body's reaction was to get rid of the boner as it needs more blood than a flaccid penis...?
Just some thoughts, I don't have a clue actually.
I believe that some causes of death, especially those that are fast and cause spinal cord damage may actually cause an erection. A gunshot to the head is an example, not sure of others. Dunno about cutting the throat though.
*in the afterlife* Oh dude you're so lucky so it turns out you can't get an erection in heaven unless you died with one so good luck with all them angels
Just some thoughts, I don't have a clue actually.