“and what would you do to me.” The sentence is unfinished in Hebrew, and is thus an anacoluthon, showing God’s great emotion at His people being conquered, mistreated, and scattered (cp. Keil and Delitzsch Old Testament Commentary). The verb is supplied in the REV for clarity of meaning.
“Are you repaying me for something I have done?” God asks the Phoenicians and the Philistines if their evil treatment of Israel is the result of some perceived wrong that Israel has done to them. The expanded translation in the REV follows the pattern in many English versions that expand the verse for clarity (cp. ESV; NAB; NIV; RSV; TNK). One of the lessons of this verse is that hurting God’s people is hurting God, and God will repay. Similarly, in the Church today, hurting Christians is hurting Christ (Acts 9:4).
“I will swiftly and speedily return your repayment on your own head.” The perceived justification for attacking and hurting Israel is not actual justification, and God says He will avenge His people. Evil people do their evil without considering that there will be a Judgment Day in the future, but there will be and things will not go well for the wicked.