(Shin no yami yori muyami ga kowai;
“Thoughtlessness is more frightening than pitch darkness”)
Being caught in utter darkness is scary, because you can’t be sure where everything is, or even what actually is and isn’t there at all. But more frightening by far is someone who acts irrationally or rashly, who lacks common sense and discernment. They can’t be counted on to do what is needed or right, and they may end up causing terrible harm whether they intended to or not. You just don’t know what they might do… only that they won’t do it with any degree of care or planning.
This time we begin at the end, with adjective 怖い (kowai), “frightening,” in conclusive form. Before that, the particle が (ga) marks this adjective as a predicate, and as its subject, the noun 無闇 (muyami), “recklessness,” “excess,” “indiscriminateness.” The particle より (yori) marks muyami as “more [scary] than” something else; in this case 闇 (yami), “darkness,” which the associative particle の (no) allows to be modified by the noun 真 (shin), “truth,” “reality” – note that in this case, the phrase 真の is taking on an adjectival role.
無闇 is an interesting word of unclear origins; most of what I can find suggests that it’s some sort of ateji – that is, kanji assigned, purely for their phonetic properties, to a previously-existing word, and that the pre-existing word may be the result of a slurring or phonetic shift acting on a longer phrase of similar meaning.
This kotowaza unfortunately does not show up in many of my usual sources, and none of the sources I can find (without an in-person visit to a library, at least, which is off the table right now) even suggest a possible source, although one blog does at least list the physical dictionary they got their definition from.
(“Shin no yami yori muyami ga kowai, to shinjiteiru kara hanchou to shite no ichiban no shigoto wa ikkansei wo mamoru koto da to kimo ni meijita.”)
[“I believe in the idea that blind action is more dangerous than mere darkness, and so I keep it firmly in mind that my primary job as team leader is to maintain consistency.”]