Meaning of してならぬ


I found this instance of してならぬ in the following passage:

> 養父を殺した騎士! 報仇の念が胸に猛った。 血の脈動と共に衝動が全身を駆け、目的を唯一事に定めて走り出そうとする。 そう **してならぬ**
> 理由が、何処にあろう。 そんなものは何処にも――

I'm well aware of the usage of してならない to mean "not being able to suppress a
feeling or action" in sentences like "そう思えてならないのだった", but this usage doesn't
seem to fit this context. In fact it seems like the sentence would make sense
if it was simply supposed to be そうして **は** ならぬ instead and it was just a
misspelling but I've encountered similar sentences a few times before so I
assume there's some meaning I'm missing but I can't find it in dictionaries.
What would be the correct interpretation of してならぬ in this case?




This sentence is a rhetorical question, "Where is the reason why I must not do
it?", i.e., "I'm sure I should be allowed to do it". そうして **は** ならぬ is more
common, but this is not a mistype, either. This type of は is optional and
often dropped in subordinary clauses. For example, 彼は学生でない sounds awkward, but
学生でない人 is safely interchangeable with 学生ではない人.

  * [Why is the topic marker often used in negative statements (ではない, ～とは思わない)?](https://japanese.stackexchange.com/q/1077/5010)

This has nothing to do with the {思え/感じられ}てならない construction.




This sentence is a rhetorical question, "Where is the reason why I must not do
it?", i.e., "I'm sure I should be allowed to do it". そうして **は** ならぬ is more
common, but this is not a mistype, either. This type of は is optional and
often dropped in subordinary clauses. For example, 彼は学生でない sounds awkward, but
学生でない人 is safely interchangeable with 学生ではない人.

  * [Why is the topic marker often used in negative statements (ではない, ～とは思わない)?](https://japanese.stackexchange.com/q/1077/5010)

This has nothing to do with the {思え/感じられ}てならない construction.

