>>94832296He turned one of the more attractive guys at his school into a lumbering oversized mutant, and one of the tech-support creatures suggested that the process may not even be fully reversible using the same technology. Morty just elephant-manned another teenager for the rest of his life out of a combination of frustration and narrowing the competition for girls at his highschool, all under the perceived justification of defending his sister because he knew that would satisfy the people in his life.
It's one of the more Rick things that Morty has ever done of his own free will. It was him not really giving a shit about consequences or actual justice and just doing what felt good in the moment. It was him using super-science to assert power and control.
What Morty did was basically worse than rape, and he did it to feel powerful, like a rapist does.