Grammar › N2 · 13 Obligation and permission
〜ないではすまない: cannot get away without
N2 · 13 Obligation and permission
Says you cannot avoid doing it, or the matter will not be settled otherwise.
Verb ない-form + ではすまない (also ずにはすまない). Often about apology, responsibility or compensation. Compare 〜ないわけにはいかない, which is about duty and 〜ざるを得ない, which is about having no choice.
Examples
こんな大きな失敗をしたら、謝らないではすまないだろう。
After such a big mistake, you can't get away without apologizing.
ご迷惑をかけた以上、責任を取らずにはすまない。
Having caused trouble, I can't avoid taking responsibility.