If harms are excused because they were not made with evil intentions then the Nazis who just liked making the numbers go up in a orderly fashion should be considered blameless.
I am pointing out how that is a absurd moral argument deriving from the rule of thumb. If anything, the corollary is the literal opposite.
Any stupidity indistinguishable from malice should be treated the same.
The usage as a epistemological rule of thumb just points out how great harm is usually caused by stupidity, intentional or otherwise, rather than intentional desire to cause the specific harm. If you aim to reduce great harms targeting just malice is not merely inadequate, it is likely missing the overwhelming majority.
You seem to be claiming that intent is not related to justice or blame, which would result in an inhumanely cruel justice system where punishments don't fit crimes.
But don't worry, I attribute your proposal to stupidity not malice
Thankfully, legal systems are not that inhumanly stupid.