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 his father would blame him for Yosef's death, which he would attribute to the fact that Yosef had taken over the Bechorah from him, 1 for which he had now taken revenge.
It was only later that Ya'akov gave Reuven's Bechorah to Yosef! Hadar Zekenim connotes that the Pasuk"u've'Chilelo Yetzu'ei Aviv Nasnah Bechoraso le'Yosef" was already known (through Ru'ach ha'Kodesh).
Why were the brothers not concerned about their father's grief
Oznayim la'Torah: It was a Divine decree that Yosef should go to Egypt tp prepare the way for Gaslus Mitzrayim
The question one can then ask is how the brothers could be taken to task for the sale of Yosef?