Great mehalech from Rav Dovid Kronglass ztz"l - Mashgiach in Ner Yisrael:
Middas Hadin [Yitzchak] truly loves man and wants to save man from the judgment of gehenom. That is why it demands judgment in this world. So לעתיד לבוא - when this discussion takes place - it is rightfully Yitzchak who will defend us in order to save us from destruction in the future. But chesed also thinks about Hashem and only as long as there is a world of avodah it asks for a sweet judgment so that it will be easy for man and Hashem will be served. But once there is no longer any avodah to do - it can say "wipe them out in order to sanctify your name".
