Albert Barnes Commentary Acts 7:25

"and he supposed that his brethren understood that God by his hand was giving them deliverance; but they understood not." — Acts 7:25 (ASV)

For he supposed. This is not mentioned by Moses; but it is not at all improbable. When they saw him alone contending with the Egyptian, when it was understood that he had come and taken vengeance on one of their oppressors, it might have been presumed that he regarded himself as directed by God to interpose, and save the people.