Re: How is it happening now?
Legally, that's why you have trials, because things aren't always black and white and judges and juries have to assess the situation on a case-by-case basis. Common example: you killed someone in an ambush, but he'd been pursuing and shooting at you previously (meaning the aggravating circumstance of lying in wait is countered by the mitigating circumstance of immediate threat to life). It's up to the courts to determine if the loss of life was the result of deliberate action (murder), negligence (manslaughter), or some other circumstance, and if so, how much is the accused responsible for the circumstance (did the accused act willingly or was coerced, threatened, or otherwise forced into a no-win situation like the Trolley Problem). Were there other people involved? One common variation of the Trolley Problem can be called Guerillas In The Village (instead of you in a trolley, it's you encountering it's a bunch of militant guerillas about to wipe out a village and deciding for variety to let the rest of the village off if you execute one of them).
That's also why you have inquiries when tragedies hit (the inquest into the late Phillip Hughes is in the news right now), to see if things were simply fate (the inquest seems to be heading in that direction) and where things could be improved in future (perhaps there will be research into better neck protection if it doesn't hamper batsmen too much).