What it means
ないではすまない expresses that a person or organization cannot avoid taking a required action, usually because of responsibility, wrongdoing, or the seriousness of a situation. It conveys a strong sense that failing to act would be unacceptable or would leave the matter unresolved. It is mainly used in formal writing, serious statements, and situations involving social or moral responsibility. The pattern follows the negative form of a verb, but the overall meaning is affirmative: the action must be performed.
Usage note
ないではすまない should not be confused with ないといけない, which can describe an ordinary practical obligation. This pattern usually implies that circumstances, responsibility, or consequences make avoidance impossible.
How to use it
重大な事故を起こした以上、会社は被害者に補償しないではすみません。
Since the company caused a serious accident, it must compensate the victims.
重大な事故を起こした以上、会社は被害者に補償しないではすまない。
Since the company caused a serious accident, it must compensate the victims.