It is important to challenge claims to legitimacy based purely on good intentions regardless of outcomes. But surely we can do this without pulling the dishonest rhetorical trick of asserting that all bad outcomes are intentional (which is what "purpose" implies)? Something like "judge a system by its outcome, not its intent"?

You are conflating intention on the part of individuals with the societal system which creates conditions in which those individuals perceive themselves as doing good despite the fact that they are serving the needs of the system. Recommended reading: https://theanarchistlibrary.org/library/ted-kaczynski-the-sy...