Why was he so distraught when he discovered that Yosef was no longer in the pit?
Rashi and Targum Yonasan: He was concerned about his father's grief.
Moshav Zekenim, Hadar Zekenim, Ba'al ha'Turim: He was afraid that he would be blamed for Yosef's death, which they would attribute to the fact that Yosef had taken over the Bechorah from him, 1 for which he had now taken revenge.
Only later, Yakov gave Reuven's Bechorah to Yosef! Hadar Zekenim connotes that the verse "uv'Chalelo Yetzu'ei Aviv Nasnah Bechoraso l'Yosef" was known (through Ru'ach ha'Kodesh).