וְעַתָּ֣ה ׀ לְכ֣וּ וְנַֽהַרְגֵ֗הוּ וְנַשְׁלִכֵ֙הוּ֙ בְּאַחַ֣ד הַבֹּר֔וֹת וְאָמַ֕רְנוּ חַיָּ֥ה רָעָ֖ה אֲכָלָ֑תְהוּ וְנִרְאֶ֕ה מַה־יִּהְי֖וּ חֲלֹמֹתָֽיו׃
Now, let us go and kill him, and throw him in one of the pits. And we will say 'A bad wild animal ate him.' And we will see what will be of his dreams."
The brothers said that they would be the ones to tell Ya'akov that Yosef was eaten by a wild animal.
We then see the sequence. They put him in a pit, Midianites take him and sell him. Then, the brothers dip his coat in blood, and present it to Ya'akov saying, "We found this. Do you recognize it?" It was actually Ya'akov who said that a wild animal ate Yosef.
It seems that the brothers really didn't know what happened to Yosef, but they planned on selling him (or did they actually sell him?), and then they make it look like he was killed and torn apart, somehow. So, why didn't they complete the plan and tell Ya'akov directly that Joseph was eaten by an animal?