I would argue in the former's case that intent still applies. Someone clearly set up the mechanism with the intent to kill - thus, they would still be the killer.

[ nerd. ]

For the latter... mm, prior knowledge would be the place to start. "Who would know enough about practical physics to predict where the spear would fall, or who would have the technical knowledge to construct the machine." Proving it, however, would be a lot harder. But not impossible. Nobody is perfect enough to completely destroy all evidence.