He should go to jail.
He should go to jail for a long, long time—possibly until he dies! He should sit in jail, and rot, and no one should visit him, except maybe sometimes Jose Peralta, the guy who now has his old seat.
If going to a literal jail forever isn't in the cards for Monserrate, though, he should probably set out to entomb himself in a prison of the mind. His poor girlfriend should be taken in by family members and deprogrammed. And Monserrate should move to a filthy hovel and drink himself to restless sleep every night. Maybe he can become addicted to pills? His only companion should be a black-and-white television set that doesn't tune in any channels. Then happily, legally married gay couples who love and respect each other very much should move in and gentrify his neighborhood and he should probably be priced out of his hovel and end up in an even shittier hovel somewhere where the buses never run anymore because of people like Hiram Monserrate. (He should have had to sell his car a while ago, obviously.)
Time should pass and he should eventually become old and infirm and nobody should care or notice except sometimes someone brings up that one asshole politician who violently attacked a woman who loved him, but no one should be able to remember his name. And then he should die alone.
That's what should be next for ol' Hiram Monserrate!