Without a clear set of laws things get messy really quickly.
We do allow for subjective peer input in a trial, that's exactly why the jury exists.
But to say that someone shouldn't even be put in front of a jury just because you think they broke the law with good intentions is undermining the purpose of law. Who are you to decide what was a "benefit to the public"?
There were people who thought that Guantanamo was a "benefit to the public" so should they be allowed to torture people?
There were people in the NSA who thought that mass collection of metadata from phone companies was a "benefit to the public" so maybe we shouldn't investigate those things.
That's not how I want the law to work. We should investigate all of those things, regardless of what you think is beneficial, under a strict set of rules of law. And we use democracy and international order to shape that set of rules.
If you think this is crazy and oppressive you should look around at how non-western countries handle these things.