SUBOTICA, Serbia, Sept. 15 - When the Setets family set off this morning to take their 13-year-old daughter to school, they got a shock. As they left their small, two-room house on the outskirts of Subotica, in northern Serbia, they found a 15-inch kitchen knife imbedded in their front door. Alongside it, someone had sprayed the Serbian word for "death" in red. Further to the right was "Drop dead, Hungarians." Such ethnically motivated attacks by Serbs against people like the Setets (pronounced sheh-tets), members of this country's 300,000-strong Hungarian minority, have become rife, souring relations between Serbia and neighboring Hungary....