Um ,is that cause it’s illegal to murder?
I think he's referring to the legal ownership of the firearm, regardless of the murder.
Either the firearm is "illegal" somehow (I'm thinking "stolen" or sold across state lines w/o involvement of a FFL), or the possessor is barred.
Keep in mind that NJ has asinine gun laws. The hoops that legal gun buyers have to go through are substantial. It is difficult and expensive to be a legal gun owner in NJ; and near impossible to be legally bearing arms outside of ones home. If all Booker is saying is that NJ's model is okay, he's not admitting that open or concealed carry is okay.