I'm having trouble understanding Rashi on Genesis 37:20
When Joseph came towards his brothers, they plotted to kill him. They said, (Gen. 37:20):
וְעַתָּה | לְכוּ וְנַהַרְגֵהוּ וְנַשְׁלִכֵהוּ בְּאַחַד הַבֹּרוֹת וְאָמַרְנוּ חַיָּה רָעָה אֲכָלָתְהוּ וְנִרְאֶה מַה יִּהְיוּ חֲלֹמֹתָיו:
So now, let us kill him, and we will cast him into one of the pits, and we will say, 'A wild beast devoured him,' and we will see what will become of his dreams."
ונראה מה יהיו חלומותיו: אמר ר' יצחק מקרא זה אומר דרשני, רוח הקודש אומרת כן. הם אומרים נהרגהו, והכתוב מסיים ונראה מה יהיו חלומותיו, נראה דבר מי יקום או שלכם או שלי. ואי אפשר שיאמרו הם ונראה מה יהיו חלומותיו מכיון שיהרגוהו בטלו חלומותיו:
and we will see what will become of his dreams: Rabbi Isaac said, This verse says: “Expound on me.” [I.e., this verse demands a midrashic interpretation.] The Holy Spirit says thus: They (the brothers) say, “Let us kill him,” but the verse concludes: “and we will see what will become of his dreams.” Let us see whose word will stand up, yours or Mine. It is impossible that they (the brothers) are saying,“and we will see what will become of his dreams,” because, since they will kill him, his dreams will come to nought. [From Tan. Buber, Vayeshev 13]
Text and translations from Chabad.org
I have trouble with Rav Yitzhak's general assumption, here. Why couldn't the brothers have been the one's making that statement? It would just be stating the result of their actions. It's as if the brothers said, "Joseph dreamt that he would rule over us. So, if we kill him, we'll see what happens to all those dreams he had. Nothing will become of them, because he won't rule over us."