The outside engineering firm argues that they didn't warn flint because they assumed flint would be using corrosion control.
When it became clear that there were serious water quality issues, flint hired another outside engineering firm to review and monitor their system and make recommendations and the second engineering firm failed to identify the lack of corrosion control as a safety issue. They basically said 'you could add phosphates to reduce complaints about discolored water, but you're still going to get lots of complaints about discolored water'.
To me it seems like a lot of the root cause was that they knew and expected the river water source would have serious cosmetic issues, but they felt forced into it. When things started going wrong they kept treating it as cosmetic issues (and it sure sounds that by count most of the complaints they had were cosmetic issues) even though the water was actually unsafe.
With two outside water engineering firms, on top of the city's own staff not catching this... I think the explanation has to be more complicated than someone being criminally negligent.
So, for example, suppose I'm a driver who takes a bribe so that I expose my passenger to a sniper somehow. I had no desire for my passenger to die - I just wanted the money, but - in some legal systems anyway - it can be argued that I intended the natural consequences of my actions.
Alternatively, even if I have not "intended", I have certainly gone beyond negligence. The commonly-recognized middle level of criminal responsibility is _recklessness_:
https://en.wikipedia.org/wiki/Recklessness_(law)
There's something inbetween, which
As dreadful as it was-- that choice hardly seems likely to be a crime. Negligent sure, but where is the criminal intent? "muhaha I'll make everyone sick!" hardly.
In an event like flint I'd expect the only crimes to be attempts to cover it up, not the event itself. ... and it seems like the the desire to prosecute the people "at fault" caused them to give immunity to the people who actually committed crimes.