Learn Russian: Мой брат был в бешенстве из-за ошибки. - My brother was in a fury because of the mistake.

The phrase в бешенстве uses the preposition в plus the prepositional case to describe an intense emotional state. In Russian, placing a person 'inside' an emotion (literally 'in a fury') is a highly natural way to express a temporary, overwhelming state of mind, which often sounds more expressive than just using an adjective.

What is the dictionary form of бешенстве, and how does it decline here?

The dictionary (nominative) form is бешенство (fury, rage, or rabies), which is a neuter noun. Because it ends in -о, it takes the standard -е ending in the prepositional case. It shares a root with the adjective бешеный (mad, furious, rabid).

Why do we use из-за here instead of потому что for 'because of'?

из-за is a preposition used to point directly to a noun as the cause of something, and it requires the genitive case (changing ошибка to ошибки). You would use потому что only if you were introducing a full clause with a subject and verb, such as 'because someone made a mistake'.

Could I use the preposition от instead of из-за for the cause?