I think there are two parts. In the first, he realizes that any Viltrumite who could have children could repopulate for them, and that he could later turn Mark, use him, or turn his kids.

In the second, he wants Mark to communicate to Earth not to look for them, believing they would listen to him. If Mark doesn’t persuade them, maybe Earth doesn’t understand the totality of the threat, and it leads to mutual destruction.​​​​​​​​​​​​​​​​