Wow. What an intense situation to be in - this sounds so stressful.

I think I would tell my husband. I might not get very specific, but I would want to tell him enough just so he knew that the friendship with the kid had to be over. I might tell him that the kid confessed a crush on me and leave out the death part, IF I was pretty sure the kid was not dangerous.

I would completely cut off the kid. You can't help him. He has to help himself.

That's just what I would do, not really knowing the particulars like you do -- I hope whatever you decide to do works out.