"If Looks Could Kill" is the name of a movie and a song, here the phrase is interpreted in the literal sense. if bad appearance would be lethal, the person in question would be the equivalent of a powerful open bolt, blowback-operated submachine gun.
while the previous explanation of incestual desires of royalty may have been well-intentioned, Salt and Pepa are most likely actually teasing us by referencing (and almost repeating) the chorus of Prince's controversial 1992 song "Sexy MF."