ずにはすまない
cannot avoid doing; have no choice but to; must (do)
N1
Meaning
cannot avoid doing; have no choice but to; must (do)
Formation
Verb (ない-form) − ない + ずにはすまない
*する → せずにはすまない
*来る → 来（こ）ずにはすまない
Explanation
ずにはすまない is used to express that a certain action is unavoidable due to moral, social, or situational pressure.
It does not mean obligation imposed by rules (like ～なければならない), but rather:
a sense of responsibility
conscience
natural consequence of one’s actions
The speaker feels that not doing the action would be unacceptable, either socially or ethically.
This grammar is commonly used in formal or written Japanese, such as explanations, reflections, news articles, and JLPT N1 passages.