What happens if no beings choose to act on one's bad karma?

Kamma is only one of the five Niyamas. Five Niyama's are Citta, Dhamma, Utu, Biju, and Kamma. The operation of Kamma is hard to understand in certainty. If Arhant does not exhaust of all his Kamma in this life all residual Kamma become non-operative.

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The suttas (SN 12.37) say [the results of] old kamma are to be felt [as feelings] & not identified with (not acted upon). This will dissolve the results of old kamma. This is why the suttas (AN 6.63) say the Noble Eightfold Path destroys/ends kamma.

As for Moggallāna, according to mythology, he was attacked because he made the Jain religion look bad when he reported Jain followers had poor rebirth. This is just a worldly matter and unrelated to any kamma of Moggallāna because, as an Arahant, Moggallāna would not have identified with any past actions & present painful feelings & therefore would not have suffered when he was attacked.

Many people hated the Buddha & wanted to kill him. That people hated the Buddha is not related to the Buddha's kamma. The Buddha has no "kamma" in the ordinary sense of the word.

MN 117 explains the ordinary meaning of "kamma" is connected to "asava" ("defilements" ) and "acquisitions" ("upadhi"; "attachment"). Arahants are free from "acquisition" thus are free from "kamma".

I believe that beings have the choice to act or not act on one's bad karma.

It seems like a good idea and safest to assume that the consequences of one's bad karma will necessarily be severe.

In short one can say that at some point it became determined that he would experience assault.

If a car is on a course of collision, the eventuality of collision at some point becomes inevitable due to the circumstances like ìt's velocity.

Analogically events become determined for this or that being due to the prior development of circumstances.

You can find the answer for your question from a Jataka called "Matakabhatta-Jātaka". Herewith I've quoted the section of the Jātaka which may answer your question.

"In times past, brahman," the goat began, "I was a brahman who taught the Vedas like you. I, too, sacrificed a goat as an offering for a Feast for the Dead. Because of killing that single goat, I have had my head cut off 499 times. I laughed aloud when I realized that this is my last birth as an animal to be sacrificed. Today I will be freed from my misery. On the other hand, I cried when I realized that, because of killing me, you, too, may be doomed to lose your head five hundred times. It was out of pity for you that I cried."

"Well, goat," said the brahman, "in that case, I am not going to kill you."

"Brahman!" exclaimed the goat. "Whether or not you kill me, I cannot escape death today."

"Don't worry," the brahman assured the goat. "I will guard you."

"You don't understand," the goat told him. "Your protection is weak. The force of my evil deed is very strong."

The brahman untied the goat and said to his students, "Don't allow anyone to harm this goat." They obediently followed the animal to protect it.

After the goat was freed, it began to graze. It stretched out its neck to reach the leaves on a bush growing near the top of a large rock. At that very instant a lightning bolt hit the rock, breaking off a sharp piece of stone which flew through the air and neatly cut off the goat's head. A crowd of people gathered around the dead goat and began to talk excitedly about the amazing accident.

Quoted from: Matakabhatta-Jātaka (Jātaka, Khuddaka Nikaya, Sutta Pitaka, Tipitaka)

The subject of Kamma is very deep and hard to understand. Only a Tatagatha Samma Samubuddha can understand.

Note: This is how I understood. I may be wrong but not Dhamma.