according to MA HUD /Section 8 a landlords responsibility is to

comply with antidiscrimination laws (they can not refuse to rent to a tenent because that tenet may be mentally /physically challenged, their race, sexual orientation....

provide habitable housing unit
follow state rent rules (dont charge more than what the state allows,meet state secruty deposit rules, meet return of security deposit rules, have rental contracts on file for each tenet....)
make legally required disclosures (report criminal activity that may be going on in the rental unit such as drug dealing, prostitution...)
dont retailiate against a tenent for reporting the landlord for discrimination, violating rental laws
follow the contracted and legal procedures for terminating tenents who may be delinquent on their rent or or other wise legal reasons for terminating a tenet in a court of law.

in the state of MA landlords do not have the responsibility to settle room mate disputes. they may evict a tenent who is committing a crime. but even then they have to go through the process of having that tenet arrested for the crime and go to court to evict that tenet.

basically disputes between room mates in MA and the USA falls under the same description of two friends having a fight, or if it gets violent it falls under the states domestic violence laws and if the problem in the rental unit can be heard outside the unit it falls under the city disturbing the peace ordinance.