My grandfather points out that the source for this is Midrash Rabbah 84: 15 in which Rav Nechamya offers that explanation for Reuven's concern. The supercommentary on Rashi, Sifsei Chachamim, quotes the Maharshal's question on this: Why bring up that reason? Why not just say that Reuven was righteous and didn't want to spill blood?
The answer can be found in the revelation of the Reuven's nature that comes through Yaakov's brachos to his sons. If Yaakov blessed Yehudah, the son who said, "ma betza" arguing to sell Yosef in order to gain some advantage, then why did he hold back a blessing from Reuven who saved his life and intended to return him to his father? The absence of the bracha indicates that even though he did save him, Reuven's intentions here were not leshem shamayim but to save himself from being blamed by his father.
My grandfather says that in his view, Yehudah merited a blessing when Reuven did not because he had the guts to speak openly to his brother and not to hide his intentions. I'd add that this fits with the idea of Yehudah meriting kingship and the trait that we see in David who speaks up honestly, as a leader should.
My granfather continues to say that the text indicates that by stressing "lehashivo el aviv," [to return him to his father] a phrase that appears to be redundant after it already said "lemaan hitzil oto miyadam" [in order to save him from their hands]. That phrase clarifies that Reuven wasn't thinking of saving Yosef for his brother's sake but for his own, that is not to be blamed in the matter.
Related posts: http://kallahmagazine.blogspot.com/2015/11/toldos-torah-reveals-motivations.html