KRS-One, the leader of this hip-hop posse, seems to be everywhere. He has written op-ed pieces for The New York Times, rapped with R.E.M., and lectured on African-American history at Harvard. But now mirroring the departure of several prominent BDP members, including KRS-One's ex-wife, Ms. Melodie several cuts on this album intentionally defy KRS-One's public image. Known for urging peace and unity in the hip-hop community, he now threatens to stomp his competition in ''Duck Down,'' the album's tough-talking first single. And, though he's not known at all for sexy (or sexist) themes, he leeringly recounts an encounter with a teenybopper mistaken for 26 in ''13 and Good,'' a song that's not only tacky but so inept you'd like to forget it's on the album at all. These departures sound suspiciously like a play for street-corner credibility after too much time on the op-ed pages, but fans can take comfort in the remaining 13 tracks, on which KRS-One sounds like his usual self. And the best news of all is that his new crew members have helped to toughen BDP's musical side. While the beats on previous BDP albums have been decidedly uneven, Sex and Violence is funky from beginning to end.