suspicion begets monsters
Proverb meaning that once you become suspicious, you start imagining all sorts of frightening things; fear and doubt create imaginary threats.
As the saying goes, suspicion begets monsters; once you start doubting, everything starts to look suspicious.
疑心暗鬼 is the four-character idiom meaning 'suspicion creates imaginary fears'; 疑心暗鬼を生ず is the full proverbial expression using the verb 生ず (to produce).
Derived from the Chinese proverb '疑心生暗鬼' (yí xīn shēng àn guǐ), literally 'a suspicious mind creates dark ghosts'. The Japanese version adds を生ず (to produce) to form a complete clause.