ないではすまない / ずにはすまない

1. 1Given the situation or one's responsibility, the action cannot be avoided and must be carried out.Ex. 1
2. 2Social or moral norms demand the action, such as apologizing or making amends, so the matter cannot end without it.Ex. 2

A stiff expression found mostly in writing and formal speech. The necessity comes from outside the speaker, such as the situation, social rules, or what others would reasonably expect, not from personal will. It often follows reason clauses like 以上 or のだから, and adding だろう turns it into a judgment that the action will be required.

• ないではおかない / ずにはおかないN1「ずにはおかない」 expresses the speaker's own strong resolve, or that something inevitably arouses others' emotions (often with the causative form); the variant 「ずにはおかれない」 means 'cannot help but.' Neither conveys obligation under pressure, so they cannot be swapped.
• 〜ないわけにはいかないN1「ないわけにはいかない」 is close in meaning and often interchangeable, but it is stylistically neutral and common in speech, emphasizing the reluctance of doing something out of social propriety or circumstances; 「ずにはすまない」 is stiffer and more formal, stressing that one cannot get away without doing it.
• ざるを得ないN2「ざるを得ない」 emphasizes having no other choice, and can also attach to verbs like 「認める」 to mean 'cannot help but judge so'; 「ずにはすまない」 stresses that one cannot get away or make amends without doing it, usually referring to duties one ought to fulfill. The two attach to verbs in the same way.
• わけにはいかないN3「わけにはいかない」 with the dictionary form means 'cannot do'; with the ない form it is close to this pattern and roughly interchangeable, but it is stylistically neutral, whereas this pattern is more formal and places stronger emphasis on not being able to get away without doing it.
• ねばならないN2「ねばならない」 broadly expresses obligation or moral 'must,' can convey resolve, and can also attach to nouns and adjectives; this pattern attaches only to verbs and stresses being forced by circumstances, unable to get away without acting. In contexts of being pressured by circumstances, 「ねばならない」 can often be substituted.
• ではあるまいかN1「ではあるまいか」 expresses a tentative conjecture ('I wonder if it isn't...'), or, as 「べきではあるまいか」, makes an implicit suggestion. This pattern attaches to the ない form of verbs and flatly asserts that circumstances make the action unavoidable. Their tone and usage differ, so they cannot be swapped.

Unlike ずにはおかない (strong determination, or an effect that will certainly occur), ずにはすまない means circumstances will not let one avoid an action. It is close to ないわけにはいかない and ざるを得ない, but it stresses that the situation or social norms will not let the matter end without the action; ねばならない states a general obligation, わけにはいかない means one cannot do something, and ではあるまいか is a tentative conjecture unrelated to obligation.