וַיַּעַן֩ רְאוּבֵ֨ן אֹתָ֜ם לֵאמֹ֗ר הֲלוֹא֩ אָמַ֨רְתִּי אֲלֵיכֶ֧ם ׀ לֵאמֹ֛ר אַל־תֶּחֶטְא֥וּ בַיֶּ֖לֶד וְלֹ֣א שְׁמַעְתֶּ֑ם וְגַם־דָּמ֖וֹ הִנֵּ֥ה נִדְרָֽשׁ׃
און ראובן האָט זיך אָפּגערופֿן צו זײ, אַזױ צו זאָגן: האָב איך אײַך ניט געזאָגט, אַזױ צו זאָגן: פֿאַרזינדיקט אײַך ניט אָן דעם ייִנגל? אָבער איר האָט ניט צוגעהערט; און אָט אױך װערט זײַן בלוט אױפֿגעמאָנט.
Rashi
וגם דמו ALSO HIS BLOOD [IS REQUIRED] — the particles את and גם extend the scope of the clause; here it implies “his blood and also the blood of his old father” (Genesis Rabbah 91:8).
Radak
וגם דמו הנה נדרש, seeing that they believed that Joseph had died after he had been sold, they felt that G’d was also holding them responsible now for having shed his blood.
Sforno
הלא אמרתי אליכם אל תחטאו בילד?. He had never intended to cause your death when he had done what he did, as you had imagined. His actions were due to the fact that he was immature, was childish.