Remember what Amalek did unto thee by the way, when ye were come forth out of Egypt;

Wesley's Notes

25:17 Out of Egypt - Which circumstance greatly aggravates their sin, that they should do thus to a people, who had been long exercised with sore afflictions, to whom pity was due by the laws of nature and humanity, and for whose rescue God had in so glorious a manner appeared, which they could not be ignorant of. So this was barbarousness to Israel, and setting the great Jehovah at defiance.

Jamieson-Fausset-Brown Bible Commentary

17-19. Remember what Amalek did-This cold-blooded and dastardly atrocity is not narrated in the previous history (Ex 17:14). It was an unprovoked outrage on the laws of nature and humanity, as well as a daring defiance of that God who had so signally shown His favor towards Israel (see on [162]1 Samuel 15; [163]27. 8; [164]30).

Matthew Henry's Concise Commentary

25:17-19 Let every persecutor and injurer of God's people take warning from the case of the Amalekites. The longer it is before judgement comes, the more dreadful will it be at last. Amalek may remind us of the foes of our souls. May we be enabled to slay all our lusts, all the corruptions both within and without, all the powers of darkness and of the world, which oppose our way to the blessed Saviour.