Radak: Kill his sons. This is due to their fathers' sins. You should fear lest the sons be evil, like the fathers. Therefore, they killed them, lest they rise and inherit the land.

Malbim: Do not leave any of them over. There are two reasons for this. (a) "Ba'Avon Avosam" - they destroyed the settled land. (b) To prevent damage in the future - "Bal Yakumu v'Yarshu Aretz." They will claim that they inherited it, for their fathers ruled over the entire world.

Rashi #1: They are haters and afflicters. This is like Targum Yonasan 'Be'eil d'Vavu.' This is like "va'Yhi Arecha" (Shmuel I, 28:16) [Radak - and "u'Fishrei l'Arach" (Daniel 4:16).

Rashi #2: They are cities. Above (17), it said that he made the settled land like a wilderness and destroyed his cities. Now it says that his seed will be finished, and city residents will return to their place, and the settled land will be full of cities.

Radak: They are cities. If [his sons] will not inherit the land, the settled land will be full of cities and settlement [but if not, not,] for they would conquer cities and destroy settled areas.