Another interpretation: he said before the Holy One, blessed be He: if His children are killed like this, where is the "thus" that You said to Abraham, "So [thus] shall your seed be" (Genesis 15:5)?
Where Is the Thus You Promised to Abraham
Yalkut Shimoni on Torah 167:5
דָּבָר אַחֵר, אָמַר לִפְנֵי הַקָּדוֹשׁ בָּרוּךְ הוּא אִם כָּךְ הוֹרְגִין בָּנָיו אַיֵּה (בראשית טו, ה) הַ"כֹּה" שֶׁאָמַרְתָּ לְאַבְרָהָם.