If Chazal view this action as sinful, undoubtedly it requires teshuvah, repentance. Horav Eliyahu Schlesinger, Shlita, derives this idea from an apparent contradiction in Chazal. He led Klal Yisrael in teshuvah. The fact that Reuven was the first to repent seems inconsistent with another Midrash that attributes this status to Kayin. Horav Schlesinger explains that Kayin repented for a sin that any human being would realize is evil.
Murder is a serious crime which demands an enormous amount of teshuvah.
The human conscience is such that a murderer expresses contrition as a natural reaction to the most horrendous act possible. In contrast, Reuven acted in a manner that did not really reflect evil. He easily could have justified his actions as the expression of a son jealous for his mother's sake. Yet Reuven realized his error, and he proceeded to perform teshuvah. This was a truly remarkable act.