I don't like this saying because it generalises too much.
For example, if you follow this line of thought you'd never investigate murderous cops simply because their cameras where turned off due to their incompetence rather than purposeful, malicious action.
I see this more as "do not assume until there's evidence of malice". Not that you should not look for evidence - basically, innocent until proven guilty. There's a tendency to judge every ignorant act as malice when, a lot of times, it's just that - ignorance/incompetence.
Not talking specifically about this case, although it may apply, we just read a twitter thread.
For example, if you follow this line of thought you'd never investigate murderous cops simply because their cameras where turned off due to their incompetence rather than purposeful, malicious action.