>presumption of innocence is on the individual, not on whether there was a crime or notThis is still wrong. A crime is an act of a person, which fulfills rather strict criteria. Both the legal theory and typical legislation [1] requires, among other, mens rea - guilty mind. Without a specific person convicted, you can't say with certainty there was a guilty mind. Thus indeed, before conviction we can't tell with certainty that there was a crime committed.
In your example of "murder", all we can say with certainty is that a person is dead. Evidence may establish a 3rd party was involved, which would indicate killing (as opposed to accident, natural causes, or self harm). There still remain three distinct possibilities: criminal murder, or the lower culpability of homicide (for example, a negligent homicide), or justified killing (for example, act of self defense). Those possibilities span the whole gamut from culpability to innocence, and warrant much different end results for the alleged perpetrator.
In case of the DieselGate here, investigating & prosecuting is the correct & practical option. We all suspect there was a crime, but there's a reason justice is dispensed via a deliberative process, rather than via mob rule.
Moreover, white collar crimes like the DieselGate usually don't have evidence as clear cut as an official memo saying, "I, hereby undersigned, order my underlings to break the law by doing X, Y, and Z.". A more realistic scenario is a series of emails & phone calls saying, "make the new engine meet the target parameters by this quarter's end", coupled with tacitly ignoring a footnote in an engineer's memo saying, "However we recommend against leaving the high performance mode enabled for production vehicles", along with a nod to a mid level manager who asks, "should we match the high performance mode to that of our competitors' car models X, Y, and Z?". In which case establishing responsibility & culpability will take some thorough work.
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[1] aside of the small set of strict liability crimes where the event/fact alone constitutes a crime