Two rough yellow stars on smudges of blue. Two billy clubs raised over a strangely haloed silhouette. Defacement (The Death of Michael Stewart) was painted by Jean-Michel Basquiat in 1983, to commemorate the death of Stewart, a young graffiti artist who was killed while in police custody. With works by Basquiat, Keith Haring, Andy Warhol, and more, the Guggenheim explores art, AIDS, and activism—the 80s in extremis. Defacement is the flashpoint. —L.J.