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.

He supposed his brethren would have understood, etc. - He probably imagined that, as he felt from the Divine influence he was appointed to be their deliverer, they would have his Divine appointment signified to them in a similar way; and the act of justice which he now did in behalf of his oppressed countryman would be sufficient to show them that he was now ready to enter upon his office, if they were willing to concur.

For he supposed his brethren would have understood him,.... From his being an Hebrew in such high life; from his wonderful birth, and miraculous preservation in his infancy, and education in Pharaoh's court; and from the promise of God that he would visit them and save them:
how that God by his hand would deliver them: wherefore he was the more emboldened to kill the Egyptian, believing that his brethren would make no advantage of it against him; but look upon it as a beginning and pledge of their deliverance by him:
but they understood not; or "him not", as the Ethiopic version reads; they did not understand that he was to be their deliverer, or that this action of his was a token of it.

For he supposed his brethren would have understood, &c.--and perhaps imagined this a suitable occasion for rousing and rallying them under him as their leader; thus anticipating his work, and so running unsent.
but they understood not--Reckoning on a spirit in them congenial with his own, he had the mortification to find it far otherwise. This furnishes to Stephen another example of Israel's slowness to apprehend and fall in with the divine purposes of love.

They understood it not - Such was their stupidity and sloth; which made him afterward unwilling to go to them.