John Calvin Commentary Acts 7:24

"And seeing one [of them] suffer wrong, he defended him, and avenged him that was oppressed, smiting the Egyptian:" — Acts 7:24 (ASV)

When he saw a certain man. Moses did not come to this spectacle by chance, but because God had appointed him to be the deliverer of His people, God would have him show this sign and, so to speak, make this beginning. For Stephen plainly expresses that he (Moses) attempted nothing rashly, but did what was fitting for him who was appointed to be a deliverer of the people, knowing that he was called to this task.

For unless God had armed him (and made him powerful), it would have been altogether unlawful for him to kill any man, however wicked he had been. It is a godly and praiseworthy deed for a man to set himself against the wicked, to defend the good against the injuries of the wicked, and to restrain their violence; but it is not for a private person to punish (or take vengeance). Therefore, it was unlawful for Moses to slay the Egyptian, except that the Lord had put the sword in his hand according to the right of his calling.

But this heroic courage and nobleness of heart was a work of the Holy Spirit; because God mightily demonstrates His power in those whom He appoints to great matters, so that they may be able to fulfill their function. In sum, Stephen means that Moses was even then offered to be the minister of deliverance when the day was near, according to the covenant made with Abraham, yet the people hoped for nothing less.