I agree with Cappuccino. I'm not really subscribing to the manhole theory, either, but "slicked off" spot or not, I don't think they were killed there.
Again, going back to the confession ('cause that's really all I've got right now that points me toward guilt), if you believe Jessie, Jason was "swinging" his knife over Byers. That would be throwing blood everywhere. I don't remember him saying anything about stabbing (as the autopsy states), but he said Jason was swinging it. So blood should be on the grass, in the trees, on them, on everything.