Federal law says that a felon is a person convicted of a crime which can carry punishment of more than 1 year, as defined by the laws of the state. In Massachusetts the following copy of the law (from the Massachusetts web site listing their statutes) specifically bars a person from owning a gun who has an adult OR JUVENILE felony conviction. The only way around this is to obtain both state and federal waiver from this disability. You have to follow the laws - which you can look up on the web site listing the laws. I doubt that anyone will give a person convicted of murder even as a teen a waiver.
The ATF does not control Massachusetts law. Federal law requires the feds including the ATF to follow the state laws. Massachusetts specifically bars ownership of guns by convicted felons, juvenile or adults.
"In the Know" apparently doesn't know how to read federal and state law. It is probably that he is illegally in possession of firearms. His emotional and insulting statements about your question suggest he may have a mental health problem. This could be a danger to society if he owns firearms as he states.
" (d) Any person residing or having a place of business within the jurisdiction of the licensing authority or any law enforcement officer employed by the licensing authority or any person residing in an area of exclusive federal jurisdiction located within a city or town may submit to such licensing authority or the colonel of state police, an application for a Class A or Class B license to carry firearms, or renewal of the same, which such licensing authority or said colonel may issue if it appears that the applicant is a suitable person to be issued such license, and that the applicant has good reason to fear injury to his person or property, or for any other reason, including the carrying of firearms for use in sport or target practice only, subject to such restrictions expressed or authorized under this section, unless the applicant:
(i) has, in any state or federal jurisdiction, been convicted or adjudicated a youthful offender or delinquent child for the commission of (a) a felony; (b) a misdemeanor punishable by imprisonment for more than two years; (c) a violent crime as defined in section 121; (d) a violation of any law regulating the use, possession, ownership, transfer, purchase, sale, lease, rental, receipt or transportation of weapons or ammunition for which a term of imprisonment may be imposed; or (e) a violation of any law regulating the use, possession or sale of controlled substances as defined in section 1 of chapter 94C;