If Joe previously made his desire to harm Simon public, and then runs him down in the street, how hard a time would he have proving it was an accident? When a guy runs over someone he’s been ranting about hating on facebook (for example), I can easily see a prosecutor thinking it’s more than a coincidence.
If Joe had previously and publicly threatened Simon, then I’d imagine defense would probably go for a plea deal on that one (even if it was in fact an accident), unless there’s some kind of hard scientific evidence that it couldn’t have been on purpose. No jury is going to believe that it was a coincidence if there was a prior threat.
Until we have mind-reading devices, proving mens rea will always be about making reasonable inferences from the available evidence. If A harms B, then a prior threat from A to B is powerful evidence of mens rea.