In fact, he had expected that after Monica chose to be with Michael, Michael would do a lot of things to keep Monica by his side. Discrediting him and ruining his reputation was one of them. Michael was very smart, and by doing so, he could make Finn hate Monica even more and widen Finn's distance from Monica. It was true that Michael had destroyed their relationship.
He could even guess what Michael would do next.
Michael would be a good person.
In Michael's relationship with Monica, he would be a good person who would comfort Monica and move her.
Slowly, Monica's heart would waver, and she would choose to be with Michael.
That was the outcome of the three of them.
Even if the person Monica loved was him, even if Monica was just helping Michael, Monica would be with Michael in the end. From the moment she chose Michael, that was the destined outcome.